Bryan Caplan  

Does Parental Divorce Cause Offspring Divorce?

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Children of divorce are more likely to eventually get divorced themselves. But why? Earlier behavioral genetic work concluded that, contrary to popular platitudes, the transmission mechanism is heredity, not environment. As Judith Harris put it:

A twin study of 1500 pairs of adult identical and fraternal twins analyzed their and their parent’s marital histories... The analysis churned out that: about ½ of the variation could be attributed to genes shared with twins or parents. The other ½ was due to environmental causes. But none of the variation could be blamed on the home the twins grew up in. Any similarities could be fully accounted by the genes they share.
A new paper by a bunch of researchers (including Eric Turkheimer) challenges this result using a "children of twins" design. Key idea: You gather data on twins and their children. Then you can see, for example, what happens to their offspring if one twin gets divorced and the other doesn't.

The main results from this paper are very different from Harris'. While genetics still matters...

The use of the Children of Twins Design revealed that parental divorce (or risk factors specifically associated with parental marital instability within twin families) accounted for approximately 66% of the initial, unadjusted estimate of the intergenerational transmission. Because our design allowed us to control for genetic and unmeasured environmental selection effects (for the twin parents) as well as measured characteristics of both parents, this is perhaps the strongest evidence to date that parental divorce directly causes an increase in offspring divorce.
Four caveats:

1. The effect size is pretty small. When you look identical twin pairs discordant for divorce (i.e., one divorced, one didn't):

Children of identical cotwins whose parents remained married had a 17.1% risk for divorce, whereas children of identical cotwins whose parents divorced had a 22.0% risk for divorce.

2. The children of twins design can't really distinguish between pure environmental effects and (unmeasured) genetic effects of the non-twin parent:

As discussed above, the Children of Twins Design does not control for genetic risk from the spouse of twins (Eaves et al., 2005).
In other words, if twins are discordant for divorce, the reason could easily be that the divorced twin married someone genetically predisposed to divorce. This in turn would make their children more likely to get divorced as a result of heredity alone.

3. The approach ignores child-to-parent causation. A difficult kid might cause his parents' divorce, then destroy his own marriage when he grows up:

Furthermore, characteristics of the offspring that increase the likelihood of divorce in both generations could also explain the relation (reciprocal influences).

4. Finally, this paper did a standard analysis of variance for the original group of twins, and reached the standard conclusion that shared environment didn't matter. In fact, the point estimate of for shared environmental variance was 0%:

Twin analyses indicated that additive genetic factors accounted for 15% (95% CI [confidence interval] = 5%–19%) of the variation in marital instability... Nonshared environmental factors, environmental influences that make siblings and twins different, accounted for most of the variance (85%, 81%–90%), with a minimal role of environmental factors that equally influenced both twins (0%, 0%–7%).
If you read this paper, you'll probably be astounded by the level of care and craftsmanship. Still, it seems pretty confusing to call something a "genetically informed" study when it can't distinguish between true environmental effects and unmeasured genetic effects of non-twin parents. If anyone's got some additional insight, I'd like to hear it.


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COMMENTS (7 to date)
Dan Weber writes:
A difficult kid might cause his parents' divorce

I'm pretty sure you're not allowed to say that.

parviziyi writes:

Three generations ago the divorce rate in USA was tiny compared to what it is today. In Irish Republic today the divorce rate is tiny compared to the UK rate. There are well-known and compelling cultural explanations for this. Bryan Caplan is being terribly glib when he speaks of cultural explanations as "popular platitudes". Too bad Bryan continues to be predisposed to overcredulousness when it comes to regression analyses. Arnold Kling has been wise and good at warning people to be very guarded about accepting the implications of statistical sausages.

Jason Malloy writes:

There are well-known and compelling cultural explanations for this. Bryan Caplan is being terribly glib when he speaks of cultural explanations as "popular platitudes".


He nowhere dismissed "cultural explanations" for differences in divorce rates. What he said was that research showed that heredity primarily explained the similarity between parents and their children in propensity for divorce.

A nongentic factor obviously explains the change in divorce rate over time. It likely has to do with different social incentives related to the economy. But the similarities between parents and children are still due to genes.

Babinich writes:

"genetically predisposed to divorce"

Wow, that was quick. Not unforeseen but I am amazed at how fast this card was pulled from the deck.

"characteristics of the offspring that increase the likelihood of divorce in both generations could also explain the relation (reciprocal influences)."

Hardly; it's the characteristics of the parents that lead to divorce.

8 writes:

I learned it from watching you!

parviziyi writes:

Epidemiology is a very poor science. The similarity between parents and their children in propensity for divorce is due to cultural heredity, not genetic heredity -- that's the commonsense view. The epidemiological studies indicating the contrary shouldn't be given credence because when you get deep into the innards of the studies you find they're very poor science.

Jason Malloy writes:

The similarity between parents and their children in propensity for divorce is due to cultural heredity, not genetic heredity -- that's the commonsense view.

The "commonsense view" is contradicted by all genetically informed studies. Propensity for divorce follows biological, not shared environmental, similarity. This is true for just about any trait; whether we model it from twin studies, kinship studies, or adoption studies, the true source of variance in behavior is largely between nonshared environment and heredity. Home environment accounts for little to none of the variance.

I very much doubt you have any real familiarity with or worthwhile objections to behavior genetics.

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