In contemporary times, twin research still remains popular, providing means for comprehending the most profound genetic determinants of human behavior (Plourde, 1986).
Distinctions between Twins: Outcomes of Twin Research
Fraternal twins are also known as dizygotic twins, and they hold exactly 50% of their genes with each other. While they are not as similar as identical twins for understanding genetic factors, they do result in interesting findings in comparison with identical twins. Fraternals may be likened to first-degree kin, minus the similarity in age as do identical twins. Twin research are dependent on both fraternals and identicals. These investigations work under the assumption that if heredity was a stronger influence than the environment, then identicals ought to exhibit greater similarity than identicals (Plomin et al, 1997). This is an instance of the “heritability coefficient’s” being critical: gauging what degree a particular trait is in comparison to others’ under one characteristic is attributable to genes (Olson et al, 2001). Such coefficient is assumed to take on a higher value for identical twins than for fraternal twins. However, it is practical to suppose that identical twins may also exhibit distinct phenotypes while possessing the same genotype. Differences may thus be an outcome of their distinct experiences and environments that in turn mold their cognition, affect, and behavior – and their personalities in general (Hughes et al., 2005).
Twins and Differences in Brain Structure
Hulshoff et al (2002) have conducted a research on the genetic effects on differences in human brain structure in relation to health and disease. Twin research has been critiqued on the generalizability of results because of distinctions in the intrauterine and family environment, in contrast with singletons. To evaluate the twin-singleton explanation of genetic contributions on variance in brain volume, brains from 112 pairs of twins and 34 of their siblings with an average (standard deviation) age of 30.7 (9.6) years were scanned utilizing MRI. The effect of birth order, zygosity and twin-sibling differences on brain volume measures were statistically analyzed through “maximum-likelihood model fitting.” The variances were said to be homogenous in terms of 1) birth order, 2) zygosity, and 3) twin-singleton status. Regardless of zygosity, the intracranial volume and grey matter volume were significantly smaller among second-born twins in contrast with first born twins and their siblings. Moreover, white matter was likewise smaller in twins in contrast with siblings. These significant outcomes in grey and white matter between these groups ceased to be significant when rectified for intracranial volume. The lateral volume, third ventricle volume, and were almost equal between twins and singletons. It has thus been concluded that second-born twins possess smaller intracranial volumes when compared to first borns and siblings. This implies anomalous early brain development among second borns. This is supportive of the suboptimal pre and perinatal environment associated to twin birth order. Other bran volumes did not yield such significant differences (Hulshoff et al, 2002).
Figure 5. Genetic continuum of similarity in brain structure in identical and fraternal twins, and non-related individuals. Colour-coded maps show the percentage reduction in within-pair variance for each cortical region. Purple/blue=normal between-individual difference (100% of normal); green=60% of normal differences; red=30% of normal differences. F=frontal cortex; S/M=sensorimotor cortex; W=Wernicke's language cortex.
Twins and Genetic Contribution to Rate of Change in Functional Abilities
In past empirical, cross-section investigation of twins, Christensen et al (2002) have yielded evidence of an important genetic influence on functional abilities, specifically among geriatric women. It is implied that functional abilities’ rate of change could explain these outcomes and that rate-of-change phenotypes may be attributable to a stronger genetic component than “level” phenotypes (e.g., functional abilities per se). If such is the case, rate-of-change phenotypes could be more potent than level phenotypes in investigations which intend to determine particular polymorphisms pertinent to aging. Christensen et al (200) have evaluated a population-based sample in 1995 composed of Danish twins with ages ranging from 75 years or greater. The authors got in touch with the same respondents at 2 and 4 years following initial contact. Constant mean-level decreases, high within-person correlations over time, and notable heritability in the female sample were noted for functional abilities. Nevertheless, structural-equation modeling presented only a slight and negligible heritability for rate of change in functional abilities 16% (95% confidence interval: 0, 35) for females and 9% (95% confidence interval: 0, 44) for males. This investigation had a big initial sample size, high rates of participation and a measure with high validity and reliability indices for rate of change in a phenotype. Despite these, the current study still demonstrated a relatively low and insignificant genetic influence over rate of change. This twin research implies that the determination of polymorphisms that affect rate of change in functional abilities among geriatric samples may be problematic (Christensen et al, 2002).
Twins and Language Lateralization in Monozygotic Twin Pairs
There is a noteworthy ratio of monozygous twin pairs which are “discordant” for handedness. A number of these pairs exhibit mirror-imaging of some “ectodermally derived” characteristics. Both characteristics of discordant left-right asymmetry may be due to delayed monozygotic twinning at the stage when the original embryo is already asymmetrical. Moreover, the authors assert that language lateralization is associated to handedness and thus may also be modified during asymmetry development of the embryo among monozygous twins. In the study, language lateralization has been gauged through the use of a functional MRI among 12 monozygotic twin pairs who are concordant for handedness and in 13 monozygous twin pairs who were discordant for handedness. Indices for lateralization were then computed from separate language activation patterns. Correlation coefficients were likewise computer to establish language lateralization intra-pair similarity. The correlation for intra-pair language lateralization was statistically significant in the handedness concordant group, in contrast with the non-significant figures garnered for the handedness-discordant group. In the latter, five twin pairs were discordant for cerebral dominance; the remaining twin pairs of discordant handedness demonstrated marked sameness in language lateralization. The strong intra-pair correlation for language lateralization in the handedness-concordant twins implies a genetic basis for language lateralization. But among monozygotic twin pairs of discordant handedness, discordance for language dominance transpires in numerous twin pairs. Discordant language dominance may be caused by a relatively late time of splitting of the original embryo, which disrupts the normal development of left–right asymmetry (Sommer et al, 2004).
Twins and Stress Response
Twin research is ideal in that it sets aside the effects of extraneous variables brought about by individual differences; twins are brought up in the same environment and are subject to the same parental influences (Goldsmith in Plourde, 1986). Goldsmith (in Plourde, 1986) has initially done twin research investigating the influence of genes on personality and activity levels, specifically in relation to their stress responses. The outcomes of his studies indicate moderate genetic effects on these traits. In one such investigation, he undertook a comparison of identical and fraternal twins’ stress reactions. The twins were separately exposed to mild stress; for instance, a stranger entering the room. He asserts that identical twins did exhibit more similar responses than their fraternal counterparts (Goldsmith in Plourde, 1986). Twin researches such as the one mentioned establish the proposition of genetics being a strong determinant of human behavior. Moreover, they support the implicit belief that identical twins are more similar and hold a stronger, closer bond when compared to “ordinary” or even to fraternal siblings (Plourde, 1986).
Twins and Individuality In a separate investigation, Case (in Plourde, 1986) undertook a survey on 750 twin pairs’ attitudes, and inquired about their parental relationships, their feelings with their twin, and their upbringing as a twin. Sample questions included open ended items such as: "What would be the most important advice you could give to parents raising twins?" and "How do you think society can help twins develop their individuality?" The primary outcome of the study is the desire of twins to be acknowledged as unique persons. She adds that twins were inclined to share a stronger bond when compared to conventional or even to fraternal siblings. The respondents likewise expressed they have strived for autonomy and have struggled against gauging their self-esteem against the achievements of their twin. The study also presented that identical twins were inclined to be less competitive and envious when compared against fraternals. However, they claimed that they resent being treated as one entity both within their family’s circle and outside of it; in effect, they have experienced more marked difficulty when they reached the crossroads of separation (Case in Plourde, 1986).
Twins and Attitudes
A specific investigation by Haimowitz et al (2005) involved establishing the degree of heritability of attitudes among twins. Other variables of interest included intelligence and other genetic determinants which may possibly exert an influence on attitudes. The participants accomplished a questionnaire and were requested to give ratings on their 1) personality characteristics, 2) physical abilities, 3) physical attractiveness, and 4) academic achievements. The outcomes of the investigation exhibited significant differences between attitudes of the participants showed partial correlations to genetic influences. For example, there was an item on sociability. This characteristic had correlations with 5 out of 6 attitude factors that subjects had toward sociability. Athleticism attitudes were strongly associated with their self-rated athletic abilities. This study showed support for the causal model; however there are still grave issues on causation. For example, leadership attitudes appear to be strongly association with self-reported assessments of physical attractiveness, sociability, and aggressiveness – however, because of sheer number, it may not be plausible to point out direct associations between genetic characteristics and attitudes (Olson et al., 2001).
Moreover, it has been suggested that their peculiar (non-shared) experiences accounted for the greatest variance in attitudes. The variance attributable to this factor was greater than that of genetic factors and common experiences (Olson et al, 2001; Van den Oord et al, 2000). These unique individual experiences were also shown to be strongly correlated with self-ratings of physical traits and intelligence. This study implies that there are critical areas that have yet to be discovered in the realm of attitudes of twins – their etiology and determinants. It has been concluded though that twins’ attitudinal differences are influenced substantially by both genetic and environmental factors.
Jones (2005) has given a commentary to the arguments put forth by Haimowitz (2005) on the heritability of attitudes among twins. Referring to the conclusion of variance among individuals on certain attitudes that could be rooted on both genetic and environmental factors (Olsen, Vernon, Harris, & Jang, 2001), she expresses that the mean age of the respondents is 30.4 years old (Olsen et al., 2001). She suggests that at this age the twins have had a wealth of nonshared experiences which could explain the variance for non-shared environment. Moreover, she contests the conclusion that this factor only accounted for minimal variance in differences in attitudes (Jones, 2005).
She further explains that attitudes, cognitions and societal norms are formed most critically during the adolescent years and are reinforced over time (Harris, 1995). The socialization process markedly transpires through the interface that these young individuals have with their peers and support groups. Moreover, this theory of socialization upholds that these groups have strong influence on molding the personality of the adolescent (Harris, 1995). This is quite easily illustrated in the case of siblings who are reared in a common environment still develop different personalities, supporting the argument for peer group influence. In the case of twins, she expresses that they tend to stay together for the most part of their adolescent years and thus share the same peer groups. In effect, they are also more likely to develop the same set of attitudes, beliefs and norms (Jones, 2005). This seems to be more applicable for monozygous than dizygous twins, since the latter is likened to ordinary siblings. It is plausible that the variation is higher for the nonshared environment for dizygous twins because they are more likely to have distinct peer groups. One other feasible explanation could be that both monozygous and dizygous twin pairs underwent distinct life experiences that eventually caused them to have differing personalities and attitudes (Jones, 2005). Twins and Mental States By definition, mental states are composed of beliefs, intentions, and wants. As early as 4 years old, a child conventionally develops a “theory of mind”, a comprehension that objects and circumstances may be erroneously be given meaning through their own mental states. Researchers have been keen on which better explains the variations in false-belief understanding. It has been illustrated that children from big families undergo advanced development of theory of the mind. On the other hand, deaf infants of normal parents, children inflicted with autism, and those with Turner’s syndrome have significantly stalled development of such theory suggesting the role of genetics. These outcomes imply that cultural and environmental determinants both exert effects on the development of mental states (Hughes et al., 2005). To delve on this issue further, an investigation has been carried out enlisting both identical and fraternal twins. The variables which were assessed included socioeconomic status, verbal ability, and more importantly, and the theory of mind. The initial part of the assessment inquired about how capable they were to associate an erroneous or mistaken belief about a character in the stories given to them. The second portion gauged the participants’ capacities to deduce inferences and their inclination to “attribute a false belief to a belief about characters within the provided stories” (Hughes et al., 2005). A substantial portion of the variance in theory of minds of twin pairs was an outcome of uncommon or unshared environments. It is also interesting to note that the strength of influence was accounted for by common (i.e. shared) environments, verbal abilities, and genetics. In addition, twins who belong to highly competitive and conflict laden families are more inclined to develop theory of minds. While this investigation does point out to the strong impact of the “nurture” factor, it is by no means a reason to discount the equally strong influence of genetics on mental states (Hughes et al, 2005). Genotype-Environment Interaction An investigation has been carried out utilizing a part of the identical twins who were enlisted in Bergeman et al’s (1988) Swedish Adoption/Twin Study of Aging (Bergeman, Plomin, McClearn, Pederson, & Friberg, 1988). The authors were keen on investigating the association between twin genotypes and phenotypes who were not raised together. This was an inclusion criterion in the Swedish investigation. Bergeman et al (1988) asserts the following: “One twin's phenotype should be the biggest indicator of the other twin's genotype, because the study examined the experiences of pairs of twins who had been separated their whole lives.” The similarity between the twins ought to be associated with their genetic make up since they did not have common environments to begin with.
The investigation has been drafted to assess the personality characteristics of extraversion and neuroticism among twins. In addition, the characteristics of impulsivity and monotony avoidance, family environment and socioeconomic status were likewise measured. Utilizing these data, Bergeman et al (1988) yielded three conclusions about the genotype-environment relationship.
The “genotype-environment interaction” is a term used in twin research, pertaining to the “potential for people with different genetic makeup to respond differently toward the same external situation (Bergeman et al., 1988).” It represents a critical construct in twin research since the genotype-environment interaction may also be applied inversely. That is, it may also assess how people with exactly the same genotypes would react distinctly given the same stimulus or environment.
A specific genotype-environment tagged Type I, suggests that the environment has stronger effect on individuals with a genotype for low scores on a particular personality trait. For example, participants who have yielded low genotypes for extraversion would also garner low scores on extraversion if they gauged their families as having high control and organization, in contrast with those who have high genotypes for the traits. The latter exhibited that high extraversion characteristic regardless of the perceived degree of familial control (Bergeman et al., 1988).
The next type of genotype-environment interaction, Type II, is contradictory to Type 1. It asserts that persons who possess genotypes that cause them to yield high scores on a particular trait are influenced by their environments. On the other hand, those who have genotypes that lead to lower scores were not influenced by their environments. For instance, a person who has a high genotype for impulsivity will enhance this characteristic when exposed to a conflict-laden environment (Bergeman et al., 1988).
Lastly, the Type III genotype-environment interaction has been deduced from an animal research, specifically conducted with mice and not from the Swedish investigation. This type transpires when the environment exerts an impact with genotypes resulting them to have higher scores on characteristics as well as persons who have genotypes that cause them to have lower scores on traits. For instance, an environment which has specially high parental control will constrain the exhibition of this phenotype. On the other hand, a tolerant environment will permit the genotype to surface as a marked phenotype (Bergeman et al., 1988).
Integration: Explaining Twin Similarities
Identical twins, even when brought up in different environments are assumed to exhibit striking similarity in behavior, more similar than their fraternal counterparts. This supports the genetic perspective to the issue. However, there are certain arguments put forth supporting the environment stance; that is, that similarities are probably accounted for by learning and common environmental influences. The following represent the four major assumptions made by the evolutionary approach to twin research (Beetle, 2005).
Learning absence in womb. One of the implicit errors is the assumption that birth as a milestone is the starting mark for learning. If this principle is held as true, then it follows that any similarities between identical twins who have been separated at birth will be completely attributed to genetic factors. On the contrary, there is sufficient empirical proof indicating that the unborn child learns as early as the first months of conception. Stimuli encompassing visual, auditory, and gustatory cues are among the first that he is exposed to, even before he is able to see. At certain times, the fetus will “jump” when a loud noise is heard; there are instances when parents can make their unborn baby “kick on cue.” This early, the unborn child is also capable of sensing different moods of the mother, feeling anxiety, health, hunger, depression, intoxication, rest, comfort and stress. For instance, the fetus of a heroin addict will be born as addicts themselves and would require medical assistance through withdrawal. In the same vein, during conception, if the mother placed vanilla or garlic in her diet, the child would prefer laced milk over normal milk (Houtilainen in Beetle, 2005). The fetus is also capable of voice recognition, and shall cherish the sound as an infant. The Mozart effect has also been noted to enhance mental and musical abilities. Among twins, the intrauterine environment has accorded the same foundations for future development – that is, even if they are separated at birth. That identical twins are more similar than fraternal twins implies more determinants from learning (Beetle, 2004).
Equal environment. Yet another implicit assumption is that identical and fraternal twins will be subject to the same morals, expectations and experiences since they shall be raised under the same environment. However, if there still remains more similarity among identical twins, then this suggests a strong genetic factor. The error in such assumption lies in the fact that people around twins are inclined to treating identical twins in a more similar manner than fraternal twins; thus, there is a greater probability of similarity between them because of receipt of more similar stimuli. Moreover, because of physical semblance, twins may have a stronger inclination to emulate each other deliberately or inadvertently. Still another reason for the greater similarity of identical twins lies in the fact that they always have the same sex, while fraternal twins may not necessarily be so. A lack of comprehension in these issues will cause one to exaggerate the role of genetic influence in twin similarity (Joseph, 2003; Horwitz et al, 2003).
Separation implies separation. Still another assumption is that the separation of twins at birth necessarily implies stopping them from having common environmental conditions that may then lead to overlaps in learning. However, it is often glossed over that identical twins are often left to the care of another relative or someone who has been approved of by their parents. Thus, this means that they hold the same values and beliefs as the twins’ biological parents. The selection of the caretakers is not done haphazardly. Moreover, any means of communication between identical twins, through their relatives or pen pal letters, is an avenue for the fusion or synergy of ideas. Separated twins will most probably seek things they do similarly and engage in them. Nurturing bonds and discovery of soul mates whom one can converse with is a prize in itself. Beetle (2005) further shares that twins may imitate each other’s mechanisms immediately following a get together. Even an unsuspecting researcher who came upon the rare examples of true separation could unintentionally put examples of similarity into the minds of the separated twins. Numerous researchers also question that these influences have been properly negated. The combination of true separation, exclusion from family contact and belief systems, and researcher independence is rare.
Similarity comes from pre-wired brains. A final assumption stems from the belief that identical twins have pre-wired brains and instincts in contrast with learning from similar phenotypes. While similar behaviors could result from either scenario; however it is critical to acknowledge which mechanism is accountable for such similarity. Beetle (2005) asks whether it is their genetic make-up or learning that has stronger influence in the process.
Another factor that I think is important in behavioral development is the foibles of the internal clock, and the diversity of the feedback loop that reaches from the cortex to the pleasure centre. All of these are physical features from which learning could occur. Physical features which granted are coded by the DNA, but that is different to saying the neural pathways are already fixed in the brain, or that DNA codes for instinct and behavior, to keep you forever bound and troubled. Humans should acknowledge the association necessary with their environment and body, as then they could improve the ability to learn and mentally be stimulated by the novel (Beetle, 2005).
The sameness of identical twins separated at birth may be attributed to factors including “fetal learning, a desire for bonding and soul mating (leads to mimicry), and predictable learning induced by the similarity of environment and bodily structure.” A direct validation of the accuracy of this proposition is “if inherited, then show me the inherited instinct or mind module in a baby, rather than claiming inheritance after those instincts are learnt and solidified in children and adults.” This has never been presented, which reinforces the fact that evolutionary psychology does both have any foundation and may still be in the process of maturing as a science. In contrast, the learning and the alignment and consolidation of neurons into distinctive patterns has been sufficiently investigated empirically (Beetle, 2005).
Overall, the studies put forth above engaging twin research illustrate that there remains to be distinctions even among the most similar individuals. There are equally strong factors from both genetics and the environment that are accountable for molding our personalities, behavior, cognition, and attitudes. In reviewing these empirical studies which have covered a wide array of topics, it is apparent that the make-up of the human being is both complex and diverse. There are certain instances where genetics seems to be the more dominant component in this interplay, while there are cases when the environment seems more have a foremost role. In the end, these same factors account for the distinctions between twins’ personalities and psychopathology.
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