Rapid human evolution

It is commonly accepted among evolutionary psychologists (at least in the Santa Barbara school*) that "our modern skulls house a stone age mind". As Tooby & Cosmides put it,

natural selection... takes a long time to design a circuit of any complexity. The time it takes to build circuits that are suited to a given environment is so slow it is hard to even imagine -- it's like a stone being sculpted by wind-blown sand. Even relatively simple changes can take tens of thousands of years. The environment that humans -- and, therefore, human minds -- evolved in was very different from our modern environment. Our ancestors spent well over 99% of our species' evolutionary history living in hunter-gatherer societies. That means that our forebearers lived in small, nomadic bands of a few dozen individuals who got all of their food each day by gathering plants or by hunting animals.

Similarly, Edward Hudgens explains,

Evolutionary psychologists downplay the possibility of significant cognitive evolution in the 10,000 or so years since the advent of agriculture (a period of time known as the Holocene) for reasons of both science and political correctness. Scientifically, 10,000 years (500 generations) is not much time for natural selection to act, and it certainly is not enough time to evolve new, complex adaptations—sophisticated mechanisms coded for by numerous genes.

New research just released in PNAS has the potential to undermine these claims fatally. John Hawks and his colleagues argue that human evolution accelerated very rapidly in the last 40,000 years. The abstract:

Genomic surveys in humans identify a large amount of recent positive selection. Using the 3.9M HapMap SNP dataset, we found that selection has accelerated greatly during the last 40,000 years. We tested the null hypothesis that the observed age distribution of recent positively selected linkage blocks is consistent with a constant rate of adaptive substitution during human evolution. We show that a constant rate high enough to explain the number of recently selected variants would predict (1) site heterozygosity at least tenfold lower than is observed in humans, (2) a strong relationship of heterozygosity and local recombination rate, which is not observed in humans, (3) an implausibly high number of adaptive substit utions between humans and chimpanzees, and(4) nearly 100 times the observed number of high-frequency LD blocks. Larger populations generate more new selected mutations, and we show the consistency of the observed data with the historical pattern of human population growth. We consider human demographic growth to be linked with past changes in human cultures and ecologies. Both processes have contributed to the extraordinarily rapid recent genetic evolution of our species.

(See also: John Hawks's two blog entries on his study, and Reuters' report)

*The Santa Barbara school of evolutionary psychology is the best known type of EP, its foremost exponents are Leda Cosmides, John Tooby, Donald Symons, David M. Buss, Steven Pinker, Margo Wilson and Martin Daly. (It's so called because Symons, Tooby & Cosmides are at UC Santa Barbara).

Active parents raise active children: the authors reply

Recently, I wrote a rather scathing blog entry (see also my follow-up post) and a more measured "rapid response" to a BMJ (British Medical Journal) article that argued parents socialize their children into being physically either more active or more sedentary. Now the authors have replied to my criticism with a rapid response of their own. (Actually, the response was published almost a week ago - oh the dangers of going on holiday!). Suffice it to say I don't think Mattocks et. al. have neither met my criticism or understood it fully, so I'm going to respond to them both here and at the BMJ website. (Submissions to BMJ's rapid response system aren't peer-reviewed but are approved by an editor, so there's no guarantee my response will in fact be published. I'll link to my response if it does get published).

A quick recap of the study: the authors (Mattocks et. al., 2007) wanted to determine which early life variables (before age 5) affected the physical activity of children at ages 11-12 (which was objectively measured with accelerometers). They found that (among other things) "children are slightly more active if their parents are active early in the child's life and suggest "helping parents to increase their physical activity... may promote children's activity." It's clear from the quotations (and the rest of the study) that the authors were thinking solely in terms of socialization. As I pointed out in my critical blog entry (which I recommend you read if you haven't done so), there's an obvious possible confound here: genetics. Since (1) all behavioral traits are heritable (Turkheimer, 2000) and (2) children inherit 50% of their genes from each parent, "genetic factors are always possible confounds when relating parenting style (or other parental behavior) to outcomes in children". Consequently, because genetic factors were not controlled for, Mattock's et. al.'s study does not distinguish between the relevant possible causal hypotheses and therefore fails to add very much to our knowledge.

The authors' reply to my criticism, I think, amounts to the following: 'controlling for genetic factors is really hard' (first paragraph), and (2) 'not all the hypotheses we tested are confounded by genetic factors' (most of the second paragraph). Let's take these replies in turn. The literature about the determinants of physical activity is certainly not one of my specialities so I don't have an opinion about whether or not it's possible to control for genetics at the present time. Let's grant, for argument's sake, that Mattocks et. al. are correct: that it's not possible to control for genetic factors. What follows? Does assuming this proposition at all support the truth of their finding concerning the link between active parents and active children? Clearly not - the fact that we cannot control for a confound manifestly (and unfortunately) does not make it causally inert. Because genetics is a possible confound (something Mattocks et. al. do not dispute in their reply), we simply can't draw a conclusion one way or another because, to repeat, their data fails to distinguish between the relevant alternative causal hypotheses. I note furthermore that Mattocks et. al. seem to have been intellectually careless - they don't seem to have considered genetic factors as possible confounds when designing the study at all. Firstly, electronic text-searches confirm my impression from reading their paper: they fail even to mention genetics. Secondly and damningly, their paper does contain a section entitled "Possible confounders" but it does not list genetics as a possible instance.

Mattocks et. al. are correct in saying that not all the variables they considered are possibly confounded by genetics. I focused on a subset of their variables and findings because I found the study through this ScienceDaily article (title: "Active Parents Raise Active Children") and because the authors themselves emphasize the correlation (what they regard as causation) between maternal physical activity during pregnancy and early life and children's physical activity later in life (see the conclusion of the abstract). Furthermore, it seems a majority of the study's positive findings are at least possibly confounded by genetics, even if in some cases a particularly plausible causal mechanism is absent. (The sum total of the positive findings were that activity at 11-12 was 'modestly associated with': "mother's body mass index before pregnancy, parents' smoking status during pregnancy, mother's age at birth of the child, mother's physical activity, parity, and season of birth.")

In short, genetics is a possible confound (a fact that remains unaltered whether or not it's possible to control for it), but, despite this, the authors didn't even try to control for it and happily drew causal conclusions in the absence of controls. My criticisms stand.

Bibliography

Mattocks, C., Ness, A., Deere, K., Tilling, K., Leary, S., Blair, S.N., Riddoch, C. (2007). Early life determinants of physical activity in 11 to 12 year olds: cohort study. BMJ (British Medical Journal). DOI: 10.1136/bmj.39385.443565.BE

Turkheimer, E. (2000) "Three Laws of Behavior Genetics and What They Mean," Current Directions in Psychological Science, 9(5): 160-164.

Evolutionary psychology as an “extraordinary claim”

Channeling David Hume, Carl Sagan famously said, "extraordinary claims require extraordinary evidence". As Richard Dawkins notes in the Afterword to Buss (2005), evolutionary psychology [EP], oddly, is often thought to make extraordinary claims meriting strong skepticism. Now, clearly, to accept the proposition that telepathic communication is occurring in a particular case, we would require extraordinary evidence and "demand multiple replications under ultrarigorous, double-blind controlled conditions, with a battery of professional illusionists as skeptical scrutineers and with a statistical p-value less than one in a billion" (2005: 977). On the other hand, the claim that, say, two people can communicate via a telephone or by exchanging letters would elicit practically no skepticism. In between these extremes are claims that one ought to be somewhat skeptical about and require a good deal (but not an extraordinary amount) of evidence before they can be accepted provisionally. Examples of this class include the proposition that smoking causes cancer (when it was first posited) and the claim that, say, cocaine has epigenetic effects on the hippocampus. The difference between the cases, it would seem, has to do with a priori plausibility: the extent to which the proposition under consideration fits in with everything else we know about the world. So where on this spectrum does evolutionary psychology fall?

Dawkins makes a convincing case that many critics of EP incorrectly place it on the telepathy end of the spectrum when it belongs in the middle, much nearer to the plausible end. Evolutionary psychology, notes Dawkins, "amounts to the exceedingly modest assertion that minds are on the same footing as bodies where Darwinian natural selection is concerned" (2005: 978). Indeed, it is the opposite conclusion – that human psychology is exempt from the forces that govern the rest of the natural world – that requires extraordinary evidence. It is the proposition that humans are unique, and somehow not continuous with the natural world, that is a priori implausible given everything else we know. Dawkins makes the same argument with respect to modularity: since "modularity is a universally good design principle which pervades engineering, software, and biology" and since it "is such an obvious way to run any complex operation, we should positively expect that the mind would be modularized" (2005: 978, emphasis added). The controversy over modularity of mind is therefore often so heated because its opponents are far more skeptical of it than they ought to be.

While I think Dawkins is exactly right when it comes to the many of EP’s critics, it is only fair to note that there is a group of people who don’t make the mistakes he discusses. Some cognitive scientists and cognitive psychologist accept the mind is a product of the brain and agree the brain evolved by natural selection, but then deny an evolutionary perspective is illuminating. (see this blog post). While we should expect an evolutionary perspective to offer insights, it is possible to adopt a functionalist perspective (a "design stance" in Dennett’s parlance) without being explicitly evolutionary. (Indeed, a good deal was discovered about human physiology using this method long before the publication of Darwin’s theory). Nonetheless, it is still clear that this sub-class of EP's critics ought to locate particular EP propositions – say, that female mate preference varies with menstruation cycle – in the middle of the skepticism spectrum, not at the telepathy end. Sometimes it seems that even these critics – such as the blog entry liked above – fail to do so and I can't help but detect antipathy bordering on bias.

Bibliography

Dawkins, R. (2005) “Afterword,” in Buss, M. (ed.) Handbook of Evolutionary Psychology (Hoboken, NJ: John Wiley & Sons)