Study Review on Twin Studies and Behavioral Development Between Twins

Behaviors arise intrinsically as well as an extrinsically from an individual, twin studies can help us identify the environmental association; as well as understanding how genes and shared environmental influences can come together to create certain processes. In this way, twin studies have not outlived their usefulness, these studies can go beyond heritability and are powerful tools for examining the genetic and environmental influences on individual differences of behavior. This is notably considerably difficulty when trying to make causal associations, twin studies have many possible confounding variables that are relevant to the results of the study.

A good example of the great way twin studies can help us understand personality, is with political views among individuals (Johnson et al 1998). When looking at twin studies, it is apparent that any difference in the democracy and voting behavior of identical or monozygotic twins cannot be attributed to genetic confounds nor any aspect of the familial environment they share. Such as their socio-demographic status, their parents’ or guardians' political attitudes, or parental/guardian voting records; meaning the difference must be due to non-shared environmental influences among the twins. Implying genetics and shared environment may have less to do with political beliefs than the experiences of the individual.

Given that genetic influences routinely are involved with behaviors the importance of twin studies lie in that they can be used as quasi-experimental test of environmental explanations. The major benefit of twin study designs is they make it possible to control genetic and shared environmental backgrounds without much difficulty. Making there no need to specify all the mechanisms involved, making it possible to isolate and test for the presence of environmental effects of interest. Twin studies have continued through the years to provide opportunities to identify causal effects with appropriate genetic and shared environmental controls. Though the existence and pervasiveness of a familial influences on behavior means that sectional bias is always a concern for any study of causal effects on environmental circumstances.

In most twin study research, they do not refer to twins as fraternal or identical, but as dizygotic and monozygotic. Whereas one results when there are two separate eggs which are fertilized by separate sperm, resulting in 50% shared genetics on average, the other results when a single fertilized egg divides and they share all their genes. They do this to avoid past misclassification which yielded incorrect analysis of the resulting estimates of genetic and environmental influences on traits of interest.

Meike Bartels in 2015 did a meta-analysis twin study of the genetics of well-being, defined as level of happiness and quality of life (life-satisfaction). Reviewing 30 past twin studies on the topic and comparing their result to that of each other, for a better analysis of genetic effects on well-being. Most of the studies analyzed used the classic two design of MZ and DZ twins with same-sex and opposite-sex pairs, two were men only, five were only same-sex twins, one makes no distinction only refers to MZ and DZ twins, four included reared together and reared apart twins, and eight were extended twin designs; six of which used non-twin siblings. Analysis of the data from these studies estimates around 0%-64% range of heritability, meaning that the influence of their genetics could be anywhere from not influencing them at all to influencing their behavior up to 64%. Specifically, it was found that with life satisfaction and well-being the influence is around 32%-36% respectively. Providing a more robust estimate of the effects genetics can have on well-being. Bartel summarized that genetic factors contribute significantly to differences in well-being, and concluded genes explain about 35% of the difference between twins.

M. Bartels and D. Boomsma also conducted with other researchers’ a study on well-being in 2009. Assessing subject well-being with four different measures, general quality of life, satisfaction with life, quality of life at the moment, and subjective happiness. Participants were adolescent twins and their siblings age 13-28, 5,024 subjects from 2,157 families were assessed. The results showed that adolescents scored higher on subjective well-being than young adults, and that MZ twin correlations were high than DZ twin correlations. Researchers also estimated that the genetic influences on all measures was at about 40%-50% influence, and that the remaining influential difference was due to non-shared environmental influences. So, it can be determined that while it each individual has a genetic determination of their well-being influences of the environmental factors (non-shared; unique to each individual) is also very important. If we can break this down, it may be possible to identify what environmental factors can make some people happier.

K. Lang, R. McCrae, A. Angleitner, R. Riemann, and W. Livesley in 1998 tested the heritability of P. Costa and R. McCrae’s revised edition of the NEO-Personality Inventory in a cross-cultural twin study. Using Canadian and German twin samples, 183 MZ and 175 DZ Canadian pairs and 435MZ 205DZ German pairs. Showing genetic and environmental influences of the same magnitude for both samples at around 25%-65%, for most facets. Each facet of the NEO-PI was broken down into its base and six other facets which fall under it. Neuroticism also measured anxiety, hostility, depression, self-consciousness, impulsiveness, and vulnerability. Extraversion also measured warmth, gregariousness, assertiveness, activity, excitement seeking, and positive emotions. Openness to experience also measured fantasy, aesthetics, feelings, actions, ideas, and values. Agreeableness also measured trust, straightforwardness altruism, compliance, modesty, and tendermindedness.

Conscientiousness also measured competence, order, dutifulness, achievement striving, self-discipline, and deliberation. It was also found that correlations between MZ twins was higher than DZ twins. Their study supports that facet-level traits are more than just part of broader factors of personality, that they are distinct constructs which have their own genetic or biological grounding. Indicating personality is complex comprised of a few broader trait factors and a large number of narrow traits.

H. Eysenck in 1990 investigated the genetic and environmental factors that influence individual differences on three major dimensions of personality, psychoticism, extraversion and neuroticism. Eysenck analyzed six studies to provide more reliable evidence on the genetic influences on personality change. Together these studies investigated about 32,600 twin subjects, one study had 500 MZ and DZ twins, second had 860 sets of twins, third had almost 4,000 pairs of twins, fourth had 12,898 twin pairs, the fifth was focused on reared-apart and reared-together twin sets, and the sixth had 14,288 co-twins (one twin not both). It was concluded that there is a great importance of genetic factors that determine differences in personality.

For some measured aspects of personality evidence points to different genes operating at different stages of development, much like with sexual maturation but shown with personality dimensions. Much like evidence that was found in a Swedish study, which implies there’s a significant interaction between different sexes and what genetic effects there will be on personality. Eysenck also found that someone’s familial environment only accounts for a trivial influence on personality differences among individuals. This is supported by another finding that MZ twins who are separated at birth are more alike in personality than MZ twins who are not separated.

C. Bergeman, H. Chipuer, R. Plomin, N. Pedersen, G. McClearn, J. Nesselroade, and R. McCrae in 1993 did a similar study to Eysenck (1900) study but instead focused on the other three components of NEO-PI (experience, agreeableness, and conscientiousness). Participants were 82 pairs of MZ twins and 171 pairs of DZ twins reared apart from one another, and 132 pairs of MZ twins and 167 pairs of DZ twins reared together. Results showed similar findings to other studies strong genetic influences on personality, but little shared environmental influences on personality. Openness and conscientiousness showed moderately high genetic influence on differences in personality, with little shared environmental influence. Though agreeableness was the opposite having little genetic influence but large shared environmental influences on personality. Personality may have biological origins but the significances in someone’s personality come from their personal life, their social and cultural worlds.

H. Goldsmith and I. Gottesman in 1981 looked to determine the origins of difference in people’s behavioral styles. Behavioral ratings were assessed for 350 twin pairs on their temperament, they were in groups on 8 months, 4 years, and 7 years old. Results showed non-familial influences accounted for half the observed difference in personality. For the 8-month olds the only factor which seemed significantly (moderately) affected by genetics was their activity factor. For 4-year olds IQ showed greater familial influences, significant genetic effects were found for task persistence and irritability factors. For 7-year olds some evidence was present for genetic effects on activity adjustment and fearlessness factors. The researchers concluded that activity level correlated from infancy to 4 years’ assessment, and that it is a moderate indicator of IQ and attention span. Genetics factors have a notable role in the differences observed of infants and young children.

N. Markovitch, K. Luyckx, T. Klimstra, L. Abramson, and A. Knafo-Noam in 2017 examined the influences on identity exploration and commitment of early adolescence. They studied 571 eleven-year-old twins, 85 MZ twin pairs and 167 DZ twin pairs completed the Dimensions of Identity Development Scale (Luyckx et al 2008). It assesses five dimensions’ commitment making, exploration in-depth, ruminative exploration, identification with commitment and exploration in-depth (which has two subscales). MZ twins were more similar to each other than DZ twins on all dimensions but exploration in-depth subscales. Supporting that some measures of personality are in fact, in part, influenced by genetics; researcher estimated this effect at about 18%-45%, with the rest of the difference coming from non-shared environmental effects at an influence of about 55%-82%. Only one of the exploration in-depth subscales showed anything different with no genetic effect but a shared environmental effect (21%) for exploring commitments through discussion with others. Genes and environmental effects should both be looked at when researching identity.

L. Baker and D. Daniels in 1990 researched non-shared environmental influences on the personality differences of adult twins. Participants were 161 MZ twin pairs and 74 DZ twin pairs over the age of 18. The researchers looked to identify what the importance of the twins’ different experiences would have on their personality. They used the Sibling Inventory of Differential Experience (SIDE; Daniels & Plomin, 1985) to retrospectively gather data on the different experiences the twin pairs had while growing up, as well as getting scale measures of affect and personality. This study showed great evidence for genetic differences in the SIDE scale, while still also showing significant differences in personality and affect among MZ twin pairs reflecting pure environmental influences on behavior. Also demonstrated was a decrease in differential experiences across genetic relatedness, meaning from adoptive to biological there are noticeable differences among siblings of varying degrees of relatedness. Differences between MZ and DZ twins are more substantial than between adoptive or biological siblings.

P. Vernon, K. Jang, J. Harris, and J. McCarthy in 1997 looked to determine environmental predictors of personality difference. They look to assess the extent of the influence that non-shared environmental factors have ones the variance of personality. Participants were 93 MZ twins: 74 female pairs and 19 male pairs, 50 pairs of DZ twins: 41 female pairs and 9 male pairs, and 66 pairs of non-twin siblings: 51 female pairs and 15 male pairs. Results showed that differences in individual responses to some environmental measures are partially due to genetic factors. Personality differences between MZ twins, DZ twins, and the non-twin siblings can be predicted by differences in their environmental perceptions. Such that with a number of personality traits, participants perceptions of their family and background environments play a significant role in the differences of personality present between different kinds of siblings.

Lastly, that character correlations between one's personality and the environmental measures present are attributed to non-shared environmental factors entirely. Shared environments have little to do with differences in personality which may be environmentally rooted.

K. McCartney, M. Harris, and F. Bernieri in 1990 conducted a developmental meta-analysis of twin growing up and growing apart. The researchers analyzed 103 research papers on developmental changes in personality and intelligence for monozygotic and dizygotic twins, to calculate extent of changes in their similarity over time. Results indicate that there's a general tendency for twins to grow apart as they grow up, or rather they become less similar overtime. The researchers found that activity-impulsivity and masculinity-femininity have substantial changes over time between all twins. As well as that other domains like anxiety and task orientation showed more similar changes between MZ twins only. Data indicated that the genetic determinations on IQ decrease with time, and for IQ and activity-impulsivity the importance of shared environment decreases over time; while simultaneously non-shared environment increases, but only for activity-impulsivity. Lastly, it was found that while all twins showed a similar pattern of dissimilarity overtime MZ twins appear to be more dissimilar overtime in some measures of personality, it was hypothesized that this is because they have a greater need to appear different as they are more likely, than any other siblings, to be compared to one another.

D. Newman, A. Tellegen, and T. Bouchard Jr in 1998 examined the individual differences in ego development of adult twins who were reared apart. Participants consisted of 45 pairs of monozygotic twins and 28 pairs of dizygotic twins, in all twins had been raised apart before being reunited as adults. The researchers estimated a broad heritability for ego development at 46%, though the researchers accept that the character variance in adult levels of ego development had substantial genetic and environmental influences.

As results indicated that reared apart twins had similar trait levels of ego development which could not be attributed to their similar cognitive ability or verbal reasoning, and must originate from factors the siblings do not share (non-shared environment). But the researchers do admit that shared family environment does appear to make a small contribution in variance of ego development of adults, though rarely at a magnitude high enough to reach statistical significance. The results found also lend credence to the theory that ego development and cognitive development are separate functional domains, does the exact nature of the relationship between ego development and cognitive ability is still unclear.

S. Hershberger, R. Plomin, and N. Pedersen in 1995 studied the reliability, stability, and shared genetic influence of traits and meta-traits among twins. Meta traits and traits are distinguished by metatraits measuring individual differences and construct relevancy and traits measuring individual differences and construct extremity. There were 24 traits and metatraits that were examined on 157 pairs of identical reared together twins and 95 pairs of identical reared apart twins, 211 pairs of fraternal reared together twins and 228 pairs of fraternal reared apart twins. Results showed reliability and stability of metatraits was lower overtime as compared to traits. Results indicate that 15% of metatrait character difference can be attributed to genetic influences, 5% to shared rearing environmental influences, and 80% to non-shared environmental influences. Researchers concluded that meta traits can have reliability and stability that is respectable depending on what character aspect is being researched, such that metatrait scores can have the same amount of confidence as a trait scale score, providing an additional view of personality.

K. Kato and N. Pedersen in 2005 investigated personality and coping of twins reared apart and twins reared together. The present study is part of an ongoing longitudinal design, consisting of same-sex twins who are separated before age 11 and reared apart and a sample of twins who are reared together. These participants samples were matched on gender age and country of birth, and the present data consists of 1339 individual twins; 58 pairs of MZ twins reared apart, 101 pairs of MZ twins reared together, 147 pairs of DZ twins reared apart, and 140 pairs of DZ twins reared together.

Results found significant gender differences in the environmental and genetic influences on coping scales. Differences in coping scales in women were due to genetic influences more so than for men, high correlations of genetic influence was found for problem solving, turning to others, and openness to experience; as well as between extraversion neuroticism and avoidance. With the exception of turning to others for men, all of these associations were either moderately or substantially due to genetic influences. There was limited influence of shared environment but substantial influence of non-shared environment. Except for turning to others and avoidance in women which had moderate but significant shared rearing environmental influence as well.

The results of the study imply that coping and personality, while both distinct constructs, have a systemic association for women. Which is very important implication when considering the fact that both coping and personality (neuroticism in particular) can be important predictors of health. Researchers concluded that although the present study is cross-sectional meaning causality is not obvious, it does provide implications of genetic intervention of the covariation between personality and coping.