Studies of offspring of identical twins

Since publishing on the laws of behavioral genetics (Turkheimer 2000, 2004), Turkheimer, his colleague Robert Emery, and their students have analyzed the similarity of offspring of monozygotic twins to clarify the relationship between parental traits, especially divorce, and behavior of their offspring.  Turkheimer (2008, 4) describes the logic of their analyses:

[I]f a genetic propensity to be aggressive makes parents more likely to get divorced, and those same genes when passed to the children make them more likely to be aggressive on the playground, then one will observe an association between divorce and playground aggressiveness that will not really be a causal consequence of divorce… But in identical twin parents… none of the differences between the children can arise from differences in the genes of their twin parent, so if the children do differ, we can (almost…) rule out a genetic explanation of the association.

Conversely,

Suppose poor families are more likely to be divorced than well-off families, and children raised in poor families are more likely to be delinquent. [We could] observe an association between divorce and delinquency that doesn’t have any causal relationship to divorce. But twin parents share their family history of poverty, so if the children of the divorced twin are more likely to be delinquent than the children of the non-divorced twin, the parental poverty isn’t a plausible alternative explanation…

Turkheimer follows up the parenthetical “almost” with caveats concerning, for example, the contribution of the other non-twin parent.  Not included among his caveats, however, are the following points:

  • a. shortcomings in estimation of human heritability (Taylor 2012; also previous post), presuming that heritability is what is meant by “genetic propensity”;
  • b. unreliability of the heuristic that all other things being equal, similarity in traits for relatives is proportional to the fraction shared by the relatives of all the genes that vary in the population (Taylor 2012; also previous post);
  • c. heritability (“genetic propensity”) as a measure of similarity in the trait does not translate in any direct way to hypotheses that invoke underlying genes or genetic factors (Taylor 2010); and
  • d. asymmetry in conceptualization of genetic and environmental (or social) factors—the latter are measurable; the former unknown and potentially heterogeneous (Taylor 2010; also previous posts).

Turkheimer (2008, 5) reports “a rich variability of outcomes”—some indicating a genetic association, some an association with the social factor, and some ruling out hypothesized associations.  We do not know, however, how much this variability of outcomes is generated by the unreliable heuristic (issue b).  Yet suppose that, for the purposes of discussion, we had results accompanied by an analysis of sensitivity to variation away from the unreliable heuristic value or, even, that the heuristic were dispensed with because we had data on the necessary classes of relatives (Taylor 2012).  Could we then interpret similarity or dissimilarity of offspring of monozygotic twins as a “reflection of the environmental and genetic developmental processes that underlie complex human behavior” (Turkheimer 2008, 5)?  My answer: Not readily.  Whereas Turkheimer’s phrases “genetic propensity” and “genetic explanation” suggest an equivalence or a direct translation between measures of similarity in the trait and hypotheses that invoke genetic factors underlying the trait, this is not so (issue c and “Gap 2” in Taylor 2010).  “Genetic explanation,” moreover, suggests a symmetry with environmental or social explanation, but this is not so either.  The asymmetry can be seen by examining the first scenario that Turkheimer uses to describe the logic of the analyses and an alternative, then considering what interventions to modify the developmental processes follow from the analyses.

The first scenario is the one in which there are no grounds for a “genetic explanation” of the association between divorce and the outcome, aggression of child.  This scenario is summarized in table 1.  (Notice that the non-measured, unknown genetic factor or composite of factors contributing to the similarity of the first pair of twins for the trait—here, aggression—is shown as distinct from the genetic factor contributing to the similarity of the second pair of twins given that the analysis cannot show the factors to be the same or similar.)  To reduce the incidence of aggression seen in children of divorced parents, it would be plausible under this scenario to focus on policies to reduce divorce without reference to genetic factors or genetic relatedness.

Table 1.  Analysis of offspring of monozygotic twins: Behavioral outcome rules out a “genetic explanation” of the association between divorce and the outcome, aggression of child.
H4

Notes: In any study offspring will come from several twins, but the number shown to illustrate the logic of the scenario.  Black/white cells indicate presence/absence of the factor.  Diagonal hatching downwards denotes offspring of any one of an identical twin pair are not identical in their genetic factors because there is a second parent and, for the same reason, offspring of parents that include the different identical twins are not identical in their genetic factors. Diagonal hatching upwards denotes twin parents are not identical in the measured factor because heritability is less than 1.

Now imagine a new scenario, shown in Table 2, in which the analysis does not rule out a “genetic explanation” of the association between divorce and the outcome, aggression of child.

Table 2.  Analysis of offspring of monozygotic twins: Behavioral outcome does not rule out a “genetic explanation” of the association between divorce and the outcome, aggression of child.
H5
Notes: In any study offspring will come from several twins and randomly chosen parents, but the number shown are sufficient to illustrate the logic of the scenario. Black/white cells indicate presence/absence of the factor.  Diagonal hatching downwards denotes offspring are not identical in their genetic factors to the twin or randomly chosen parent because there is a second parent and, for the same reason, offspring of parents that include one or the other of an identical twin pair are not identical in their genetic factors. Diagonal hatching upwards denotes a) twin parents are not identical in the measured factor because heritability is less than 1; and b) similarly, for non-twin parents, the hypothetical genetic factor is not expressed fully as the measured environmental factor.

What is learned about possible interventions to modify the “environmental and genetic developmental processes that underlie complex human behavior”?  Clearly, we should not focus on policies to directly reduce divorce.  But, beyond that?  We could seek to identify the unknown genetic factors associated with aggression of a parent, but nothing in the analysis rules out these factors differing from twin pair to twin pair and among non-twinned parents.  In light of that possibility, we could restrict our focus to close relatives (Taylor 2010).  For identical twins who become parents, once aggression is seen in children of one twin, we could advise the other twin to be more attentive to the issue of aggression.  That is, we could seek ways to help the parent reduce their aggression insofar as it affects the children (starting perhaps before the parents have offspring).  And we could seek ways to help the children reduce their aggression.  However, we could also advise such attention to aggression independently of knowledge about genetic relatedness and of hypotheses about underlying genetic factors.  There is no justification for thinking that, because (unknown) genetic factors have an influence, environmental interventions in the developmental processes are unlikely to succeed or a diversion of resources from measures more likely to be fruitful.  In sum, in analysis of the similarity of offspring of monozygotic twins, the environmental factors are measurable and point to interventions, but the genetic factors are unknown, potentially heterogeneous, and informative only for advising close relatives even in the thought experiment where issues a and b have been overcome. (Of course, behavioral outcomes and environmental factors may also be heterogeneous [Gatze-Kopp et al. 2012], but this does not help us interpret the outcomes of the analyses of Turkheimer and colleagues.)

References

Gatzke-Kopp, L. M., M. T. Greenberg, et al. (2012). “Aggression as an equifinal outcome of distinct neurocognitive and neuroaffective processes.” Development and Psychopathology 24(Special Issue 03): 985-1002.
Taylor, P. J. (2010). “Three puzzles and eight gaps:  What heritability studies and critical commentaries have not paid enough attention to.” Biology & Philosophy 25(1): 1-31.
Taylor, P. J. (2012). “A gene-free formulation of classical quantitative genetics used to examine results and interpretations under three standard assumptions.” Acta Biotheoretica 60(4): 357-378.
Turkheimer, E. (2000). “Three laws of behavior genetics and what they mean.” Current Directions in Psychological Science 9(5): 160-164.
Turkheimer, E. (2004). Spinach and Ice Cream: Why Social Science Is So Difficult. Behavior genetics principles: Perspectives in development, personality, and psychopathology. L. DiLalla. Washington, DC, American Psychological Association: 161-189.
Turkheimer, E. (2008). “A better way to use twins for developmental research.” LIFE Newsletter (Max Planck Institute for Human Development)(Spring): 2-5.

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2 thoughts on “Studies of offspring of identical twins

  1. Laura Surratt

    Dr. I have a question, My identical twin sister and I have children I have two boys and she has one. I told my sister that I think our sons are genetically more like brothers or 75% half brothers they are cousins. Would that be genetically correct and legally they are cousins.

    Thank you. \
    Laura M

    Reply
    1. Peter J. Taylor Post author

      Full siblings share on average half of the genes that vary in the population they come from. The cousins that are children of identical twins share one quarter. If identical twins have children with identical twins the children would be like siblings. They way I would suggest you use to see this is to imagine everyone had two chromosomes with only one gene on it, but the variant of the gene was different from chromosome 1 to 2 (e.g., AB for you and your twin, CD for the other parent of your children, EF for the other parent of your twin’s children). Given that a parent passes on only one of every pair of chromosomes to their children you can draw the 4 different combinations your children will get from their parents (AC, AD, BC, BD) and the 4 different combinations the cousins will get from their parents (AE, AF, BE, BF). Now write down what fraction of genes are the same in each of the 4×4 = 16 possibilities for two cousins. The average of those fractions works out to be 1/4 for the cousins that are children of identical twins. You can use this method to see that full siblings share 1/2 and normal cousins share 1/8. (There may be more elegant ways to explain this, but a web search shows lots of not-so-clear explanations.)

      Reply

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