November 26, 2006

Interracial beauty is due to greater symmetry?

I haven't read it yet, but at Amazon I've browsed Alon Ziv's book Breeding Between the Lines, whose thesis is that interracial individuals are more healthy and attractive, as evidenced by greater symmetry, which in turn is supposed to reflect their lower developmental instability. That is, interracials are presumed to have genomes that are better at dampening the effect of noise in their developmental system, so that the actual phenotype more closely matches the target phenotype.

But at the foundation of any chain of syllogisms must lie solid, or at least promising, empirical justification. Right now I'm working through Developmental Instability: Causes and Consequences, which covers the single phenomenon of Developmental Instability (DI) from a variety of perspectives -- quantitative genetics, evolutionary genetics, measurement of Fluctuating Asymmetry (FA), and so on. I'll be writing up a longer post on this for GNXP, but suffice it to say for now that the deceptively intuitive notion that "Lower FA = Lower Instability" turns out to be much muddier. This research program really took off in the early '90s, and by the time this 2003 volume was published, the initial enthusiasm became quite tempered. In brief, the research is pretty equivocal as regards whether or not DI reflects FA, particularly with respect to "good genes" models of sexual selection whereby choosers are purported to use a potential mate's FA as an honest signal of its DI. The latter includes the subset of studies that find an effect of symmetry on attractiveness in human faces. Later meta-analyses (including one published for the first time in this volume) show that symmetrical faces really are more attractive, but the effect size is not as large as was supposed.

An admittedly rough rule-of-thumb to follow when judging the merit of studies on the FA-DI relationship is that the earlier studies (before, say, 1998) tend to be less reliable, largely because of smaller sample sizes and lack of repeated, independent measurements of the subjects' FA -- the latter is especially important since the magnitude of variation in FA is well within measurement errors (i.e., most individuals are not pronouncedly asymmetrical). Lots more to say at a later point, but for now it's safe to say that, if interracials really are more attractive, it would only be due in small part to their greater symmetry -- assuming the latter hypothesis is true (there is currently no evidence for or against it).

PS -- the most convincing evidence that something extraordinary occurs with interracials is if their mean is above or below the means of both parental populations. However, this isn't necessary: the definition of antagonistic or synergistic epistasis (in other terms, "depression" and "vigor," respectively) only requires that the mixed mean be above or below (respectively) the value expected from additive genetic effects. So, if group A with mean IQ of 100 mated randomly and in equal proportions with group B whose mean is 70, then the mixed group's mean should be 85 if only additive (linear) genetic effects were at play. Suppose it turned out that the mixed group actually averaged 95 -- that would still be "hybrid vigor," since it's +0.67 SD above expectation (where IQ distributions have an SD = 15). Conversely, if they averaged 75, that would still be "hybrid depression," since it's lower than expectation. How this affects fitness is pretty difficult to guess -- many traits affect fitness, and in general the various races have their own strengths and weaknesses. For example, Northeast Asians score about 1.33 SD above African-Americans on intelligence and the myriad traits that are largely influenced by intelligence (such as SES), but they are probably 1 to 1.33 SD below African-Americans on height, and probably something similar for levels of gregariousness and self-confidence. And dancing ability, fuhgeddaboudit.

Fitness is always defined for a particular environment, so making general statements about how fitness is impacted by interracial status is unlikely to bear much fruit. However, testing hypotheses about in which situations interracial mating would increase, decrease, or have no bearing on fitness seems a safe bet for getting robust results.


  1. In an odd twist of science, though, we have those who make absolute statements to the effect that there is no reason to discourage interracial interbreeding, while moderate and cautious statements to the contrary are treated to ad hominem vilification.
    If nothing much is at stake here, why the passion and enthusiasm to ditch the ordinary prudence of diction among scientists?
    On p.136 of Nicholas Wade's Before the Dawn, there is mentioned that "Joel Hirschhorn of the Harvard Medical School has found that the block containing
    the lactase gene in lactose tolerant Europeans extends for about
    1 million DNA units."
    What will happen to this block
    if the carriers breed outside the group of those who also carry it?
    Isn't this a potential price that we're not being told about, when it
    is said that there is nothing to lose by such means?
    These questions are not rhetorical; I'm not sure what exactly should be expected to happen in such cases or how fast.

  2. Re: lactose tolerance -- again, even if black-white interracials were less tolerant of lactose than whites, this cost to whites would still be just one facet of the cost-benefit equation. I don't doubt that interracials will appear less fit in some ways -- "appear" since it may not actually affect fitness -- but I also don't doubt that they'll appear more fit in other ways. The net effect is too tricky to predict, I think.

    The crude rule-of-thumb is to look at what environments the two separate pops are adapted to for all relevant traits. To the extent that they're similar, the interracials likely won't be any worse off -- and may actually exceed expectation if they've inherited two distinct, optimal solutions that their parental pops have independently discovered. To the extent that they're different, the interracials will be torn in differing directions, not being optimal in either of their parents' ancestral environments.

    To take an example close to home for me, interracials of northern Euro and northeast Asian descent will likely reach or exceed expectation for IQ, since both N.Europen and NE.Asian environments have selected for higher IQ. On the other hand, the optimal personality -- or range of personalities -- may be different for N.Europe and NE.Asia.

    Suppose it pays to be more clan-ish in NE.Asia than in Individualist N.Europe: then selection will make NE.Asians more suspicious of others. At first a few genes of large effect will make NE.Asians pretty darn suspicious, though over time modifying genes will temper this to a less extreme level. As a result, you have a co-adapted gene complex responsible for current levels of suspiciousness among NE.Asians.

    But what if that complex is broken up by mating w/ more trusting N.Europeans? An interracial kid may get the uber-suspicious allele from the NE.Asian parent but not the modifying genes, so they'll appear even more suspicious & mistrusting than expectation. I don't know of any studies looking at this, and it's hard to judge others since suspicion & mistrust is a pretty private thing. But for me, my 2 brothers, and my dad, I'd say this isn't far off from the truth. (My dad's 1/2 Euro / Japanese, his sons are 1/4 Jap.)

  3. Combining the above with your thesis of symbiont/parasite manipulation sometimes yielding unexpected favorable result:
    What if some populations expanded nepotism towards moderate ranges of genetic distance, beyond cousins,
    but this turned out to be a manipulation of a lactobacteria, building on the
    1 million DNA unit block,(lacP A)
    getting it treated as if it represented the totality of the variable genes?
    This is the hard case for bio-imagination to expatiate upon;
    how can nepotism and reduced suspicion and hostility ever widen its circle in such degrees? That is, where genetic causation is to have a great role, and has to start from nothing, and not be free-rided out of its chance to exist.
    Would some heritable preference for such a block as long as the above, be enough to generate the expanded nepotism and trust tendencies, strictly regarding genetic distance,
    or would there have to be some additional push as from a microbial manipulation, such as the above-suggested?

  4. Joel Hirschhorn of the Harvard Medical School has found that the block containing the lactase gene in lactose tolerant Europeans extends for about 1 million DNA units." What will happen to this block if the carriers breed outside the group of those who also carry it?

    The existence of a "block" just means that the mutation giving lactose tolerance arose on a certain background. So the presence of this mutation is also correlated with other variation in a 1 million bp region. As time goes on, the size of the block will reduce due to recombination.

    If people with the mutation breed with people that don't, the mutation will just spread out in different populations. I'm pretty sure lactose tolerance is dominant, so that should mean more lactose tolerant people in the world (i.e. more heterozygotes). I'm not sure why this should be a "price". or a benefit, for that matter.

  5. I'm pretty sure lactose tolerance is dominant

    the european SNP which confers elevated transcription at a particular site on LCT is cis-acting. iz a gain of function.


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