Abstract
There has been a vigorous debate in recent time, about heredity versus environment and how they coalesce to create personalities, behaviors, and psychopathology that appeared unique to each person and influenced their developmental process. This study explores and used different methodological approach to lifespan development to measure and analyze the interplay of genes and environments on human behavior and underline how the interaction contributes to behavioral changes and continuity. The study planned to increase our knowledge and understanding of the interaction between heritable and ecological factors in the path of behavioral growth and patently recognize etiological mechanism that support the assumption that a particular hereditary or environmental condition ultimately lead to behavioral feature and changes during development. The study used the accessible materials on lifespan development to understand the continuous source of person uniqueness in personality development. Finally, citing evidence from various multidisplinary studies, the article concluded that shared environment significantly influenced human behavior but decline with age and the strong interaction between the two factors formed the basis of who we are and how we behave.
Keywords:
Twins, shared environments, nature, nuture, personality,genetic, ecological, continuity,interaction, attitude, human development, lifespan
Chapter 1
Introduction
There has been a vigorous debate in recent time, about heredity versus environment and how they coalesce to create personalities, behaviors, and psychopathology that appeared unique to each person and influenced their developmental process. At the same time, earlier research studies on human development shed more light on the significant effects and associations between nurture, nature and human behavior. However, this assumption further supported the general views that human development cannot complete without proper knowledge and understanding of the interaction between internal and external variables on personality. Based on this assumption, it is obvious that human behavior is an experience of a life span. Thus, our orientation about life and the way we react to the situation around us is as results of interaction between different psychosocial factors such as genetics, social norms, core faith, and attitude.
Besides, numbers of prominent scholars "have come out with evidence to support the assertions that our social world, in which we live include our family origin, school, vocation life, or even an entire community, all offer developmental options and common limitation that affect our behavior. In addition, Bronfenbrenner, 1979 and Lerner, 2002 expressed a similar position that human behavior cannot be fully understood without mentioning the changing relationship between human and the contextual environment. The milieu according to reviewed studies is the inner biological levels while outer are the social and environmental levels.
Moreover, the recent improvements and progress reported in the study of human molecular genetics has highlighted the need for further studies on how to integrate environmental measures into genomic studies, if we are to have a better understanding of the various mechanisms that support changes in human development. On the other hand, earlier studies on the charting of the human genome and the equivalent accessibility of genome-wide association analysis (GWAS) techniques has increased research activities on how to decide "genes for" particular disorders and traits. Nevertheless, research indicated that both past and present literature emphasized the significant importance of social and contextual environment on specific genetic variants in human behavior and traits.
In addition, research on Bronfenbrenner"s bioecological model (Bronfenbrenner & Ceci, 1994) also emphasized the importance of interactions between people, families, peers, school, and community characteristics in explaining the concept of personality differences in development. Therefore, most reviewed studies on life span development highlights and improved our basic understanding of the mechanism involved in normal and abnormal behavior. Thus, to understand the processes involved in analyzing differences in behavior, there is a need to look at multidisciplinary studies that measure and focus on the risk and resilience factors at many levels.
Purpose of Research Paper
This paper will explore various ways that describe the interaction between heritable and ecological factors in the path of behavioral growth and patently recognize etiological mechanism that support the assumption that a particular hereditary or environmental condition ultimately lead to behavioral feature and changes during development. The paper begins with brief overviews of the main tenets of lifespan development and explores how the concepts of nature and nurture are vital to this perspective. Moreover, the paper will highlight findings from different methodological approaches that explored the significant influence of environmental and genetic competence on behavior across the life cycle. The paper will use some of the most interesting research literature and methodological approaches in life span development to illustrate the interplay of genes and environments on human behavior and underline how the interaction contributes to behavioral changes and continuity. Lastly, the paper will focus on how the existing interaction signifies continuous source of person uniqueness in personality development.
Objectives
The main objective of this essay is drawn from the literatures on human development and it looks at evidence that support the associations between environment and genetic factors and how this interaction lead to personal differences and changes across the life span.
Therefore, the research paper aims at the following specific goals:
To analyze the interactions connecting nature and nurture as important components of behavior
To critically investigate the mediating role of genes and environments that link risk and behavior
To review the literature on different multidisciplinary studies that measure and focus on the interaction between environment and genetic factors at multiple levels of development and how this affects human behavior.
CHAPTER 2
Background Information
The assumption that race and individuality are the outcome of heredity, to say the least a biophysical phenomena, and that their interpretation is based or depends on the philosophy and process of biology has continued to gain ground and dominate debate in recent time among scholars and academician in the fields of social sciences. However, various reports show that there is a general assumption among scientist that support the causal correlation between environment, individuality and race . Consequently, research also shows that explanation of the concepts, i.e. (environment and heredity) is based on their interpretation of the developing organism. Moreover, to accept and apply the definitions to human development, reports from earlier studies show that literatures are analyzed from the following perspectives: different evolutionary theory, contributions of modern experimental genetics, and the findings of the social sciences.
However, earlier studies confirmed how the debate about nature and nurture interaction dominates and influenced most research studies on human development and formed the basis for discussion among scholars and researchers alike. Nevertheless, despite mentioning the significant influence it has on human development, recent evidence has shown that many psychologists view the issue as inconsequential and uninteresting to study. On the other hand, evidence from, the reviewed literature supported the assertion that genetic and environment factors notably influence development of human behavior.
Besides, reports from the reviewing literature show that human being is a creation of genetic and its past background, while present background offers the basis for a proper explanation of current behavior. To support the argument, reports show that the interaction between hereditary and environmental factors, formed a particular trait in a human being, therefore any specific differences associated with this trait among people or groups is linked to either nature and nurture influence. As a result, all efforts undertaken by most researchers and traditional investigations to find such factors, have yielded inconclusive answers.
Moreover, it is worth noting that the existing interaction and interconnection between the two variables (nature and nurture) will help to create the knowledge base that focus on the aspects of human personality connecting the genes, and what features is the upshot of environmental influence in human behavior. It is also worth nothing that both heredity and environment give 50% to human development. Base on this argument, the question remains which of the two factors contributed higher percentage than the other in age group disparities (Petrill et al., 2004). Besides, various scholars and research studies on life span development identified and came out with various strategies that analyzed the characteristics of human beings, this includes studies on adoption, family, and twin studies respectively, to rationalize influence of environment and heredity on development.
Thus, to explore the argument further, most research work highlights and review diverse area under discussion to analyze human behavior and psychopathology, this includes learning about person approaches to mental disorder schizophrenia. It is obvious that these studies may offer answers to the debate about the influence of nature and nurture in human development. However the fact remains that there is a need for further investigation that focuses on developing new ideas and in analyzing earlier studies.
In addition, most research pointed out that all efforts channel towards establishing the relative contribution of genetic and environment on disparities observed in traits have not achieved any meaningful accomplishment. Thus, with the exception of the issue of controlling conditions, evidence shows that most assessment focuses on the assumption that genetic and environmental interests in an additive way. This assertion further supported the general opinion of both geneticists and psychologists which explain that human behavior is based on the interaction of genetic and environmental (Haldane, 1938, Bjorklund & Pellegrini, 2002, Loevinger, 1943, Schwesinger, 1933 & Woodworth, 1941, Chiappe & MacDonald, 2005). Therefore, the level of influence of each reason is based on role carried out by the other. Based on that, it is assumed that the relative connection of heredity to difference in a specified trait, not stable, will definitely differ under different ecological conditions.
In the same way, evidence shows that in diverse inherited settings, there is a high chance that relative support of the environment will vary. Additionally, many research evidence shows that studies that measure the relative role of heredity and milieu, rarely explain how to measures such interaction. Thus, it is clear that genetics and milieu influences all human conducts and the degree of each contribution is not measurable in some trait. This assertion further confirmed why some scholars regard the debate on heredity-environment as unworthy.
Methodological Approaches
The past decade has demonstrated an increase in the number of studies that outline the link between a particular feature in the hereditary conditions of persons and their characteristics manners and features. This experience continues to show that there is a major shift from expressive and correlation strategies of the past years to new purposeful challenges that validate illustrative hypotheses. Besides, evidence shows that categorizing differences in psychological traits have lent credence to the general beliefs about significant changes in a group features following situational adjustment.
Therefore, there is an urgent need to explore various ways that explain the interaction between heritable and ecological factors in the course of behavioral growth and at the same time, clearly show etiological mechanism that supports various assumptions that any inherited or natural situation will eventually lead to a change in human trait. These assertions, though debatable, continue to dominate various research studies on human development particularly, the words, "how" of heredity and environment. In view of this argument, the following ideas and promising methodological approaches will answer the question "how "in this paper.
The Impact of Genetic factors in Continuity and Change in Life Span Development
The use of genetically informative designs to analyze human behavior has demonstrated and unravels importance of, interindividual differences in trait scores that come as a result of inherited attributes (i.e., genetics) and environmental influences. Similarly, evidence shows that the recent studies on genetically informative longitudinal designs has unravelled the genetic and environmental factors that measured occasions, these include the process that estimate and explain the intensity of the stability of each element (i.e., hereditary and environmental continuity). Besides, recent reviewed literature shows that despite the assumptions of genetic continuity, research shows that it is not ideal and faultless (i.e., reliability coefficients are constantly minus 1) from childhood to adolescence stage (De Fruyt et al., 2006; Gillespie, Evans, Wright, & Martin, 2004; Spengler, Gottschling, &Spinath, 2012).
On the other hand, most reviewed studies also show that during childhood and adolescence periods, fresh hereditary factors appeared and add to interindividual variation and sequential changes that exist in behavior. Moreover, reviewed past and recent evidences on genetic influence shows that similar to rank-order stability, genetic stability is enhanced from teenage years to middle age (Hopwood et al., 2011; Viken, Rose, Kaprio, &Koskenvuo, 1994) pending middle and late adulthood when it becomes stabilized and perfect (W. Johnson, McGue, & Krueger, 2005; Pedersen, 1993; Read, Vogler, Pedersen, & Johansson, 2006).
The Effect of Environmental Factors in Continuity and Change in Life Span Development
The results from environment studies on behavior do without question, offers evidence that highlighted that the secured inherited gap that happened in a person"s when he or she become 40yrs, is not related to the putrefaction of stability coefficients reported in older age. However, whether this assumption is convincing enough or merely suggestive is debatable among researchers.
Nevertheless, result from the reviewed studies shows how those nongenetic factors may add to rank-order continuity. Similarly, in other findings from a broad range of genetically informative studies, Johnson, Vernon, and Feiler, (2008) reported that the main nongenetic basis of person"s dissimilarity in neuroticism and extraversion is unambiguous (i.e., not distributed through relatives nurtured as one) and this according to their findings are referred to as nonshared environmental effects. Nevertheless, evidence shows that from a behavioral genetics perception, "environmental" include physiologically and biochemically intervened consequences, such as situational or ecologically triggered epigenetic influences. This report illustrates that of shared environmental factors demonstrate the disparity in permanence (i.e., environmental continuity) in term of age analyzed. However, in a similar study conducted by Gillespie et al., (2004) they reported that children who are twins and age between 12yrs and 16yrs, showed significant low coefficients in ecological continuity for extraversion (i.e. . 17 and .18 was reported for males and females respectively) and neuroticism (. 12 and .36 for male and females) respectively.
In addition, Hopwood et al, (2011) conducted a study on developmental changes commencing teenage years on the way to adulthood (i.e. 17- 24yrs) and reported natural-continuity coefficients of .36 for negative emotionality (neuroticism), .37 in support of common positive expressivity, and .39 for inherited positive emotionality in behavior, while (the last two qualities represent parts of extraversion). Moreover, their study also investigate the continuity sequence between the ages of 24 and 29, and for this interval, they reported higher ecological continuity, with coefficients varied from .56 to .60. However, in another related study, Johnson et.al, (2005) detailed 5-year environmental-stability coefficients over .70 for middle adulthood. In addition, findings from other studies show high decrease in environmental continuity in old age (Pedersen & Reynolds, 1998; Read et al., 2006).
The Genetic – Environmental Interaction
The evidence often cited in various research studies in support of genetics and environment interaction shows that there is a high correlation between life narratives, memories and experiences, and at the same time this may have cumulative effects across the life span. Accordingly, results confirmed that environment has significant consequences of interindividual disparities on neuroticism and extraversion and also enhance through age, hence, results to ecological variance (McCartney, Harris, &Bernieri, 1990). Nevertheless, research also established that an increase in interindividual difference as a result of ecological effects that rise with age, will lead to decrease in gaps that come from genetic effects. Therefore, it's worth noting that inherited assessed qualities decrease with age. Thus, the evidence shows that most studies on genetics that have measured heritability assess for diverse age cluster (e.g., Loehlin & Martin, 2001) reported no considerable disparities between age groups.
On the contrary, it was also reported that, findings from other studies that combined cross-sectional data (from different age cohorts) and longitudinal data continue to show how heritability estimation decrease with age (Kandler et al., 2010; Viken et al., 1994). Besides, recent evidence from reviewing literature confirmed that other researchers have derived various methods that ascribe various personality changes to environmental factors such as personal requirements in social roles (e.g., worker, partner, parent) and associated investments (Roberts & Wood, 2006), normative life stages (e.g., moving from one"s parental home, finishing school or a trade, leaving job due to old age), personal life situation that influence action that changes ones life (e.g., accidents, having a child or, marriage). To sum it up, research confirmed that everything being equal; personality development and change are significantly linked with age-graded social roles and correlated experiences (Roberts, Wood, & Caspi, 2008).
Therefore to buttressed the argument, similar evidence from another genetically revealing meta-analytic review study of fifteen diverse age groups showed high correlation between the level of irritability of neuroticism and extraversion and age studied .This heritability of extraversion has a propensity to increase in people in their early 40s, and later declines continue, while for neuroticism, research shows that heritability progressively decreases when people have reached adulthood. On the other hand, the model observed amid cognitive ability with age, shows differences in the genetic contributions to interindividual disparities in neuroticism and extraversion. However, various studies on cognitive abilities continually show how genetics rise with age (W. Johnson, 2010) and this further confirmed the position that genetic material and environmental effects differentially contribute to human behavior (i.e. Personality ) and the level of their competence (i.e. abilities or skills). Therefore, a boost in the level of personality trait heritability may replicate the high consequence of dynamic and reactive gene-environment correlations.
This assumption proofs that individuals actively manipulate and influence their environments (e.g., by change their friends, residences, or jobs) or exhibit social reactions that relates to their genetically predisposed traits. Similarly, evidence shows that environments and social reactions, influences the individual's personality traits. For example, persons who are genetically extroverted may see life events as more manageable and positive; nonetheless, understanding life events as manageable and constructive will definitely enhance that kind of people"s levels of extraversion (Kandler, Bleidorn, Riemann, Angleitner, & Spinath, 2012). This shows that, genetic influences to a degree build the ecological effects accounting for the mounting genetics components. Therefore, it is worth noting that the processes involved in gene-environment correlations may be the determinant factor in the development of cognitive abilities across the life span and the development of extraversion in the earliest year of life.
Cultural Studies
Another potential study about the environment and genetic influence on human development can be found in the comparative study of child-rearing practices in diverse cultures and subcultures. One of the earlier researches on culture was reported in the study conducted by Whiting and Child (1953). In the study, they make use of data on 75 primitive societies from the Cross-Cultural Files of the Yale Institute of Human Relations, to check a number of hypotheses concerning the associations between child-rearing practices and personality development. This investigation was followed up by field observations in five cultures, the results of which have not yet been reported (whiting, et al, 1954). However, evidence shows that similar surveys have been concerned with the various psychological environments provided by different social classes (Davis, et al., 1946).
Nevertheless, one of the most interesting studies is the one conducted by Williams and Scott, (1953) on the relationship between socioeconomic level, permissiveness, and motor development among Negro children, and the exploratory research by Milner, (1951) on the connection involving reading readiness in first-grade children and patterns of parent-child interaction. However, Milner in his findings reported disparity between the lower-class child and the middle-class child. Moreover, the study analyzed the disparity between the two classes of children as "a warm positive family atmosphere or adult-relationship pattern which act or seen as a motivational requirement for adult-controlled learning." However, the findings indicated that children from the lower-class see adults as mostly hostile. Furthermore, the study reported an extensive opportunity to relate verbally with adults in the family as a motivating factor. I.e. (attitude demonstrated by parents on chatting during meal time), however, findings show that lower-class parents tend to suppress and discourage such discussion, while middle-class parents support it.
Twin Studies
It has been well established that the argument about nature and nurture influence on human behavior cannot complete without exploring research on twin studies. However, recent evidence shows that most well-liked research on twin studies focuses on evaluating the resemblance between MZ and DZ twins jointly raised in the same environment or on a variable of interest (Jang, 2005). Similarly, research shows that both Identical twins, and monozygotic twins, are siblings with shared genotypes. Besides, it was established that study of identical twins serves as the best indicator that measure the significant influence of biology on traits and psychopathology in human development. For example, evidence shows that for a twin to have a dark hair or brown eyes, then the other twin will share the same characteristic of dark hair or brown eyes as well; nevertheless, studies show that the concept of identical genes perfectly match the phenotypes that forms the orientation and character of identical twins (Plomin, DeFries, McClearn, &Rutter, 1997).
Besides, earlier studies of identical twins, or dizygotic twins, shows that they shared half of their genetic traits with one another. Moreover, despite sharing 50% genetic traits, the results show that they are not as good as identical twins in interpreting the level of genetic influence, but act as a good point of reference meant for evaluating identical twins. The study of fraternal twins shows the similarity that exists in first-degree family, except that they are not sharing the precise same age, like identical twins. Consequently, twin studies typically rely on illustration of monozygotic and dizygotic twins; however, if biological influence is much more than ecological influence, then dizygotic twins would have or display psychopathology behavior similar to each other compare to monozygotic twins (Plomin et al., 1997).
In addition, this assertion further highlights the significant influence of heritability coefficient in human behavior i.e. the estimate of how individual's particular trait compares to others with similar characteristic is related to genetic materials (Olson, Vernon, Harris, Aitken, & Jang, 2001). Hence, research shows that the coefficient is significantly higher in identical twins compare to fraternal twins. Besides, research also confirmed that identical twins often express different phenotypes (outside expression of genes) for similar genotypes (genetic composition). Thus, studies explained that these attributes if noticed, represent their nonshared environments; moreover, despite identical twins sharing the same genetic composition, nonetheless, reports show that they may experience different orientation all through their lives and this actually form the personality, behavior, and psychopathology that further express and uphold their uniqueness from one another (Hughes et al., 2005).
Attitudes
In addition (Olson et al., 2001) conducted a study to determine the heritability of mindset as well as the innate variables, such as intelligence, that influence feelings and behavior among pairs of twins. Their findings show that there is significant correlation between attitudes displayed by the participants and genetic factors. The results also identified that attitudes linked to self-reported perceptions or to actions were frequently correlated. For instance, the study asked the participants to rate their trait of sociability, and the findings show that the trait was related to 5 out of 6 attitude factors subjects had toward friendliness. Also reports show high correlation between attitudes toward athleticism and self-reported athletic abilities.
Research evidence highlights that causal model was particularly sustained in these results, because physical skill (the mediator), for example, is related to attitudes toward athleticism. However, despite the general assertion on this model, evidence shows that the model is not free of criticisms, for example reports highlighted that generally, it is difficult to believe that X is the source for the occurrence of Y in all circumstances. However, it is generally assumed that approach toward leadership seemed to be connected to soaring self-ratings of physical attractiveness, friendliness, and aggressiveness. Nevertheless, despite the implication of these various factors on behavior, it is still not probable to always correctly think of direct associations between genetic traits and attitudes (Olson et al., 2001).
Additionally, earlier evidence proofs that uninsured background experiences between duos of twins played significant impacts in determining the level of attitude variances and this overriding heritable predisposition as well as collective environment experiences (Olson et al., 2001). Moreover the term nonshared environment refers to as the element within the environment that have direct influence on twin but less impact on the other (Van den Oord, Boomsma, &Verhulst, 2000). Therefore earlier studies explain that some of the nonshared environment experiences are highly related to feelings and self-reports of physical features and cleverness (Olson et al.). This debate and findings lead to the need for future research and questions about nonshared environments: why twins" behaviors and personalities are influenced by different environments? And why are some behaviors are linked to heredity, while others are not?
Theory of Mind
A study shows that beliefs, intents, and desires are very paramount to the development of human mental state. This assertion is well mentioned in various studies on human development as a theory of mind and clearly explained falsely interpretation or representation of the object and situation of a child at the age of four. However, based on these findings, research continues to search for answer on what really accounts for the variation in how children differ in false-belief comprehension between biology and environment? Though, numerous evidences proved that children from extended families developed fast attainment of assumption of mind, but hearing-impaired children born to families of hearing adult"s shows slow accomplishment of theory of mind. Though, this situation is due to cultural influences that come because of environmental influences. Moreover research also reported that children with autism disorder also exhibit impaired theory of minds, these include girls diagnosed of chromosomal disorder Turner's syndrome. However this development is linked to heritable influences (Hughes et al., 2005).
In addition (Hughes et al., 2005) conducted a study on identical and fraternal twin to test the implication of genetic and environment of theory of mind. The study explores social status, spoken ability, and last but not the least, the assumption of mind of each subject. The survey questions contained questions that examined the abilities of participant to connect a phony conviction about a character in stories provided, while the second part of the survey package contain questions that measured the subjects' skills to make assumptions and their abilities to attribute a false belief to a belief about characters in a given story .
However, findings show that the recorded variation that existed between the twin"s theories of minds is due to uninsured environments and the proportional power in decreasing order were linked to common environments, vocal abilities, and then heredity. Thus, the evidence shows that a family with twins is associated with high sense of competitiveness, in addition the more the families talk about conflicts, the faster the theory of minds. The study further confirmed that environmental influences are more significant than genetic influences in the development of theory of minds in children; however this situation does not overshadow the existence of genetic materials and role entirely (Hughes et al., 2005).
Adoption Studies
The study of adoption and how it influences human behavior and psychopathology is highly debated by researchers studying heredity and environment. Numerous evidence shows that adoption studies are very vital because they contain two sets of factors that invariably responsible for the disparities in behavior, personality, and psychopathology: natural parents and environmental parents. Similarly, evidence shows that the significant relationship that is associated biological parents and the adopted child is influenced or related to genetics. Likewise research also revealed that any association or links involving the adoptive parents, and the adopted child is typically ascribed to the environment (Plomin et al., 1997).
Schizophrenia
An earlier study on adoption shows that family environment has a less significant influence on child's mental disorder such as schizophrenia. However, in a study carried out through interviews of adopted children of biological mothers who have a medical condition of schizophrenia, and that of adopted children whose biological parents are free from any mental disorder, reports shows that there is a significant relation between adopted children of schizophrenic mothers and schizophrenia, while adopted children of parents who didn"t suffer schizophrenia show no sign of schizophrenia in life. Moreover, this supports the general assumption that stressed the point that no matter the theoretical perspectives, the particular environment that a child grew up did not influence risk for a disorder. For instance, if a child"s parent or parents have a medical condition of mental disorder, there is a high tendency that the child will experience the same risk disorder whether he or she was raised by its biological or adoptive parents (Plomin et al., 1997).
Furthermore, evidence from reviewing literature on adoption studies showed high proportion and significant correlation between adoptees of birth parents with schizophrenia, and flaunted schizophrenic-like behaviors. Most research findings on child adoptees whose real parents did not have schizophrenia or having a condition of schizophrenia themselves, shows that only a small proportion of them have a schizophrenic – type condition. Besides, review of current studies on adoption maintained similar findings and reported that a large percentage of proband adoptees showed signs of psychotic symptoms, while other evidence confirmed that little percentage of control adoptees demonstrated these kinds of symptoms. Additionally, study also long-established that the adoptees whose natural parents experienced schizophrenia will show higher chances of schizophrenia or other associated disorders if the adoptive families have low functioning. Therefore, this and other related findings continue to support the assumption of genotype-environment interaction theory, and further lay credence to the general expression that genotype is linked to the type of background environment (Plomin et al., 1997).
In addition, despite the success recorded in earlier research about genotype-environment interaction; evidence continues to indicate the difficulties encounter in answering what openly causes schizophrenia, and why it is not highly noticed among adopted children. Consequently, one of the main difficulties survives in the lack of knowledge of a genetic material that bears the disorder schizophrenia. Hence, it is new to figure out whether such genetics factor exists in human beings, and if so, results show that it is doubly difficult to comprehend at what level does this potential gene control these forms of adoption studies (Loehlin, Willerman, & Horn, 1988).
Infant Shyness
Another relevant study that examines the genetic-environment interaction in adoptive study was conducted to unravel the reasons why there is a major difference in the way infants respond to attention. Earlier research shows that some infants are openly responsive to attention; some are slow, while others are afraid and reserved. Moreover, it is difficult to deduced whether babies are shy because of their mothers do not seldom take them out, or because the shy mothers transfer the genetic shyness trait to their child. Consequently, the general procedures highlighted in this study further confirmed the significant association between the infants, adoptive and biological parents shyness, parental sociability, and parental introversion-extraversion (Daniels &Plomin, 1985).
Besides, evidence from the study conducted shows that adoptive parents were administered questionnaires to rate the levels of their child's shyness and also rate themselves on the traits listed earlier. However, report highlights that the self-reported ratings of the genes were carried out before giving birth to the infants, and the rating of the infants' shyness was achieved by the adoptive parents when the babies reached the age of two. Thus, reports on nonadaptive families showed that there is significant correlation between parents who reported high rates of shyness, low rates of sociability, and high rates of introversion and shy infants. A similar report was also recorded in adoptive families whose parents rated in the same way and this further indicates the interplay between home environment and genetics on cognitive and social development. Moreover, earlier reports confirmed that biological mothers are rated high in introversion and this also applicable to their adopted-away babies. Therefore, this and other findings confirmed the significant importance of a genetic link over family environment, and stressed the need for further research on the issue (Daniels & Plomin, 1985).
Antisocial Personality Disorder
Recent studies show that various attempts have been taken to determine any correlation between children who are at risk for antisocial personality disorder and development of symptoms in an adoptive family environment, or whether such environment will save from developing disorder's symptoms. However, research confirmed that there is high tendency that antisocial personality disorder is likely to develop in adoptees with biological risk factors (at least one biological parent had a background of criminality or antisocial personality disorder). Similarly, reports highlighted that adoptees with no history of developmental disorder will not have the symptom, despite living in an adoptive environment. Therefore research shows that the interaction between adoptive family environment and the preexisting biological risk makes antisocial personality disorder relatively common among adoptees (Roth & Finley, 1998).
Additionally, various evidence from reviewing literature shows that adoptees will develop higher possibility for antisocial personality disorder if their biological and adoptive parents are both from criminal backgrounds. However, despite this assumption research shows that many factors need to be considered before arriving at that conclusion. For example, most research studies show that there is a need to affirm or illuminate on whether this disorder is as a result of a characteristic of the biological mother, or father. Nevertheless, evidence shows that most of these studies were conducted using only information generated from the biological mother, without involving the other important segment, the biological father.
Moreover, an earlier report shows that there is no general consensus about the use of criminal background as an immediate checkmark for analyzing antisocial personality disorder in biological and adoptive parents. Although it is commonly ascribed that the presence of a biological parent's criminal background means that such parent has a medical condition of antisocial personality disorder, at the same time he or she has transferred it down to the adopted-away offspring. These results further corroborated other findings by confirming that such problem cannot be proved and that lack of a criminal background means lack of the disorder itself (Roth & Finley, 1998).
Family Studies
Research confirmed that earlier work on family studies is not as popular as twin and adoption studies, but nonetheless they are still very much relevant and applicable in analyzing the problem associated with heredity versus the environment. Beside most scholars pointed out that family studies continue to act as a benchmark and imperative factor that discover and determine the level of menace of relatives developing mental disorders that are common and affects other family members. Thus to support this assertion, case-control family studies are employed, and this includes total number of relative risk and population relative risk of a mental illness. Moreover reports confirmed that the relative risk correlate with the possibility that a family member of a person with a mental will develop a disorder than someone from a family with no history mental disorder. As a result, the report shows that population relative risk calculates roughly the level of risk that affect a person from families with mental health condition as opposed to families of a person with no history of any mental illness (Jang, 2005).
Based on various research findings, it was long-established that family studies have often served as a benchmark that determine the menace of transferring mental disorders to children within families. However, this does not totally explain the significant influence and contribution of outside factors, such as family environment and culture. Research shows that these studies are performed using molecular genetic studies, and the DNA is taken out from participants' blood samples, which further explain that the correlation between the DNA and the observed behavior is projected. Therefore, according to earlier studies, the most common molecular genetics study is called linkage analysis. This kind of study tries to locate a specific gene on a chromosome in the human body. Therefore, if a gene for a particular mental illness is being investigated, evidence shows that researchers need to identify an already-known gene on the chromosome and make it with a marker. Therefore, the location of the markers and that of the actual diseased gene is very vital: However, the closer the two are, the higher the possibility that the disease and marker genes are related or connected together (Jang, 2005).
Bipolar Disorder and Schizophrenia
A review studies on epidemiology and family studies advocate a number of resemblances between Schizophrenia and Bipolar disorders. On the other hand, family studies, mostly, propose a narrow overlapping threat for SA diagnoses and several affective disorders (Gershon et al 1988; Kendler et al 1993; Maier et al 1993). In addition, various evidences have also linked this connection with constant shared inherited risk. Earlier studies confirmed the similarities that exist between bipolar disorder and schizophrenia from the normal age of inception to the courses of the illnesses. According to the study conducted by (Berrettini, 2000) family studies, including molecular genetic studies, were carried out to determine the level of overlapping of genetic risk that exists for both disorders .On the other hand earlier studies on bipolar disorder showed that there is significant correlation between first-degree relatives of people with bipolar disorder and related mental disorders such as bipolar I disorder, bipolar II disorder, schizoaffective disorder, and recurrent unipolar disorder. However, this does not show higher risk for schizophrenia itself (Berrettini, 2000).
To buttress the above findings, reports show similar findings on first-degree relatives of people who suffered from schizophrenia. Those families are prone to schizophrenia, schizoaffective disorder, and recurrent unipolar disorder, but not for bipolar disorder. Additionally, first-degree relatives of people with bipolar disorder and schizophrenia will definitely have a higher risk for schizoaffective and recurrent unipolar disorders. These findings justified the significant relationship and overlapping between familial risk for bipolar disorder and schizophrenia (Berrettini, 2000).
Conclusion
It is obvious and evidently established in various methodological approaches on nature and nurture interact that human behavior and personality is highly influenced by genetic and environmental factors. At the same time reviewed literature on twin, adoption, and family studies offered clear evidence on how heredity and environment contributes to human behavior, personality, and psychopathology. On the other hand, various reports and analysis of twin, adoption, and family studies raised a large array of topics that support the assertion that bedrock for each human being varied in structure. For example, in some instance, the study shows that genetics seem to dominate; while in other instances, environment elucidated all. However, in most situations evidence shows that the strong interaction between the two factors formed the basis of who we are and how we behave.
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