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!


“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!

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ibn khaldun notices regression to the mean (and entropy)

too bad he didn’t get his behavioral genetics right, though. but that’s ok. he was ahead of the times. from The Muqaddimah: An Introduction to History; In Three Volumes – I [pgs. 105-106]:

“14. Prestige lasts at best four generations in one lineage

“The world of the elements and all it contains comes into being and decays. Minerals, plants, all the animals including man, and the other created things come into being and decay, as one can see with one’s own eyes. The same applies to the conditions that affect created things, and especially the conditions that affect man. Sciences grow up and then are wiped out. The same applies to crafts, and to similar things.

“Prestige is an accident that affects human beings. It comes into being and decays inevitably. No human being exists who possesses an unbroken pedigree of nobility from Adam down to himself. The only exception was made in the case of the Prophet, as a special act of divine grace to him, and as a measure designed to safeguard his true character.

“Nobility originates in the state of being outside. That is, being outside of leadership and nobility and being in a base, humble station, devoid of prestige. This means that all nobility and prestige is preceded by the non-existence of nobility and prestige, as is the case with every created thing.

It reaches its end in a single family within four successive generations. This is as follows: The builder of the family’s glory know what it cost him to do the work, and he keeps the qualities that created his glory and made it last. The son who comes after him had personal contact with his father and thus learned those things from him. However, he is inferior to him in this respect, inasmuch as a person who learns things through study is inferior to a person who knows them from practical application. The third generation must be content with imitation and, in particular, with reliance upon tradition. This member is inferior to him of the second generation, inasmuch as a person who relies upon tradition is inferior to a person who exercises independent judgement.

The fourth generation, then, is inferior to the preceding ones in every respect. Its member has lost the qualities that preserved the edifice of its glory. He despises (those qualities). He imagines that the edifice was not built through application and effort. He thinks that it was something due his people from the very beginning by virtue of the mere fact of their descent, and not something that resulted from group (effort) and (individual) qualities. For he sees the great respect in which he is held by the people, but he does not know how that respect originated and what the reason for it was. He imagnes that it is due to his descent and nothing else. He keeps away from those in whose group feelings he shares, thinking that he is better than they. He trusts that (they will obey him because) he was brought up to take their obedience for granted, and he does not knw the qualities that made obedience necessary. Such qualities are humility (in dealing) with (such men) and respect for their feelings. Therefore, he considers them despicable, and they, in turn, revolt against him and despise him. They transfer leadership from him and his direct lineage to some other related branch, in obedience to their group feeling, after they have convinced themselves that the qualities of the (new leader) are satisfactory to them. His family then grows, whereas the family of the original (leader) decays and the edifice of his ‘house’ collapses.

“That is the case with rulers who have royal authority. It also is the case with all the ‘houses’ of tribes, of amirs, and of everybody else who shares in a group feeling, and then also with the ‘houses’ among the urban population. When one ‘house’ goes down, another one rises in another group of the same descent.

The rule of four (generations) with respect to prestige usually holds true. It may happen that a ‘house’ is wiped out, disappears, and collapses in fewer than four, or it may continue unto the fifth and sixth generations, though in a state of decline and decay. The four generations can be defined as the builder, the one who has personal contact with the builder, the one who relies on tradition, and the destroyer.”

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behavioral genetics in a nutshell

from jayman!:


previously: it’s not nature and nurture…

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