behavioral genetics and twin studies

so this paper was making the rounds on twitter earlier in the week…

Demonstrating the Validity of Twin Research in Criminology [pdf]

…and everyone was like, “you have GOT to read this!” so i did. and, yeah, it’s really good! i highly recommend having a read of it.

the paper was written as a response to another paper entitled Pulling back the curtain on heritability studies: Biosocial criminology in the postgenomic era.

postgenomic? wt…? what planet do those guys live on?

anyway…Demonstrating the Validity makes many very right and very interesting points including the following:

1) a common assumption in behavioral genetics analyses is that human matings are random (they’re not — there’s a whole lot of assortative mating when humans go forth and multiply) and that assumption can lead to an underestimate of heritability estimates.

2) another assumption — the equal environments assumption (i.e. that both identical and fraternal twins share equal environments as kids) — leads to an overestimate of heritability estimates.

the authors of the paper show, however, that since these two assumptions are often found together in twin studies, that the over- and underestimates wind up more or less balancing each other out (see below — and see the paper!).

(i, of course, couldn’t help thinking that if assortative mating leads to underestimates of heritability, what would lots of inbreeding in a population do? so far, all twins studies have come from w.e.i.r.d. populations, though — afaik — so the inbreeding issue prolly doesn’t matter. but, if you should ever see some heritability estimates coming out of saudi arabia or sicily, heads up!)

other important points from the paper [my emphases in bold]:

When multiple divergent methods converge to illustrate a consistent finding, it is logically reasonable and empirically sound to accept as valid the results of the divergent methods. The primary reason for such a conclusion is the nonoverlapping assumptions underlying the various methods. This dynamic is illustrated in the convergence in findings between classical twin design studies and various molecular genetics methodologies. GCTA and CSG studies are examples of methodologies that are not subject to the same assumptions as classical twin designs and yet provide convergence in terms of the differential influence of genetic and environmental factors on a variety of behavioral phenotypes, including antisocial behavior. In direct contrast to the appraisal provided by Burt and Simons, the most cutting-edge research emanating from molecular genetics relies heavily on the findings of classical twin design studies and is continually providing empirical support for the validity of such studies.”

oh yeah!

also:

“We have shown empirically that violations of the assumptions of behavioral genetics studies do not invalidate heritability estimates. This is not a matter of opinion but a matter of mathematical evidence. Under certain conditions, our calculations and simulations revealed that heritability estimates will be slightly upwardly biased (probably no more than 5–10 percentage points). Under other conditions, heritability estimates will be downwardly biased (probably no more than 5–10 percentage points). Under the most likely condition, where multiple violations occur simultaneously, the biasing influences of assumption violations wash out, with upwardly biasing factors canceling downwardly biasing factors. Moreover, the overall pattern of findings flowing from the 61 studies examining the EEA revealed the same conclusions offered by our calculations and simulation data. Needless to say, no ‘fatal flaw’ in behavioral genetic methodologies or assumptions has been discovered and the conclusion that ‘all of these models are biased toward inflating heritability and underestimating shared environmental influences’ (Burt and Simons, 2014: 226, emphasis in original) is unequivocally incorrect.”

good stuff! read the whole thing here! and don’t miss all the supplementary materials here!

(note: comments do not require an email. identical. almost.)

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9 Comments

  1. “When multiple divergent methods converge to illustrate a consistent finding, it is logically reasonable and empirically sound to accept as valid the results of the divergent methods.”

    I don’t think that can get stressed enough!

    The same is true with genomic evidence for the existence of race (e.g., ADMIXTURE + PCA)

    “1) a common assumption in behavioral genetics analyses is that human matings are random (they’re not — there’s a whole lot of assortative mating when humans go forth and multiply) and that assumption can lead to an underestimate of heritability estimates.

    2) another assumption — the equal environments assumption (i.e. that both identical and fraternal twins share equal environments as kids) — leads to an overestimate of heritability estimates.”

    It’s important to note that the above discussion is merely theoretical. In practice, multiple lines of evidence clearly shows that the “equal environment assumption” is perfectly valid. They go over much of the evidence for this the paper, and Misdreavus also discusses it here.

    As you note, there’s plenty of assortative mating going on. That tends to bias downward heritability estimates (and upward shared environment estimates).

    Additionally, there’s the problem that most behavioral genetic studies rely on self-report, which is considerably unreliable. Using peer reports tends to boost the heritability considerably (and reduce the shared environment, when there is any).

    But, overall, great paper.

    Readers see also:

    All Human Behavioral Traits are Heritable | JayMan’s Blog

    The Son Becomes The Father | JayMan’s Blog

    More Behavioral Genetic Facts | JayMan’s Blog

    Reply

  2. “i, of course, couldn’t help thinking that if assortative mating leads to underestimates of heritability, what would lots of inbreeding in a population do? so far, all twins studies have come from w.e.i.r.d. populations, though — afaik — so the inbreeding issue prolly doesn’t matter. but, if you should ever see some heritability estimates coming out of saudi arabia or sicily, heads up!”

    One would imagine that increases the additive genetic variance. If you used a twin study, your heritability might be artificially low and your shared environment artificially high, because fraternal twins would be more similar than they would be in an outbreeding population.

    That said, there is some non-WEIRD behavioral genetic data, mostly from East Asia (Japan, South Korea, Taiwan). The results are in line as those from the West.

    Reply

  3. Svante Pääbo delivered a fascinating lecture today at the annual Nobel Conference at Gustavus Adolphus College in Minnesota.

    “Of Neanderthals, Denisovans, and Modern Humans”

    Reply

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