Category: evolution

Coyne on adaptive rumination theory (again)

A while ago I blogged about Andrews and Thomson’s *adaptive rumination hypothesis* (ARH) of depression, which holds that depression is an evolutionary adaption designed to help us solve difficult problems. I linked to two critiques of ARH by Jerry Coyne, who is clearly no fan of ARH. Coyne’s now taken his argument to the [pages of Psychiatric Times|http://www.psychiatrictimes.com/depression/content/article/10168/1575333], where he tears ARH to shreds for a third time. The main thrust of Coyne’s argument is that Andrews and Thomson employ a colloquial definition of adaptation (i.e., something that’s useful) rather than the more appropriate evolution definition:
Andrews and Thomson consider depression an “adaptation“ because it supposedly helps the sufferer solve problems. But an evolutionary adaptation is more than something that is merely useful. Biologists consider a trait adaptive only if that behavior, and the genes producing it, enhance an individual’s fitness—the average lifetime output of offspring. It is this genetic advantage, and the evolutionary changes in behavior it promotes, that is the essence of adaptation by natural selection. To demonstrate that depression is an evolved adaptation, then, we must show that it enhances reproduction.
Andrews and Thomson don’t do this, or even try. And if they did try, they probably wouldn’t succeed, for everything we know about depression suggests that rather than enhancing fitness, it reduces it. The most obvious issue is suicide, a word that, curiously, does not appear in Andrews and Thomson’s text. Statistics show that those with major depression are 20 times more likely to kill themselves than are individuals in the general population. Evolutionarily speaking, this is a strong selective penalty. Depression also appears to reduce libido and may make one unattractive as a sexual partner. Andrews and Thomson point out depression’s “adverse effect on women’s fertility and the outcome of pregnancy.“ Other health problems are comorbid with depression, although it’s not clear whether depression is the cause or consequence of these problems. Finally, studies show that depressed mothers provide poorer care of their children.
As Coyne notes, this is a problem not only for ARH, but also for a number of other evolutionary psychological accounts of depression–essentially, all those theories that posit that the depressive state *itself* is adaptive (as opposed to balancing selection/heterozygote advantage models which allow for the possibility that some genes that contribute to depression may be selected for under the right circumstances, without implying that depression itself is advantageous).

A while ago I wrote about Andrews and Thomson’s adaptive rumination hypothesis (ARH) of depression, which holds that depression is an evolutionary adaption designed to help us solve difficult problems. I linked to two critiques (1, 2) of ARH by Jerry Coyne, who is clearly no fan of ARH. Coyne’s now taken his argument to the pages of Psychiatric Times, where he tears ARH to shreds for a third time. The main thrust of Coyne’s argument is that Andrews and Thomson employ a colloquial definition of adaptation (i.e., something that’s useful) rather than the more appropriate evolution definition:

Andrews and Thomson consider depression an “adaptation” because it supposedly helps the sufferer solve problems. But an evolutionary adaptation is more than something that is merely useful. Biologists consider a trait adaptive only if that behavior, and the genes producing it, enhance an individual’s fitness—the average lifetime output of offspring. It is this genetic advantage, and the evolutionary changes in behavior it promotes, that is the essence of adaptation by natural selection. To demonstrate that depression is an evolved adaptation, then, we must show that it enhances reproduction.

Andrews and Thomson don’t do this, or even try. And if they did try, they probably wouldn’t succeed, for everything we know about depression suggests that rather than enhancing fitness, it reduces it. The most obvious issue is suicide, a word that, curiously, does not appear in Andrews and Thomson’s text. Statistics show that those with major depression are 20 times more likely to kill themselves than are individuals in the general population. Evolutionarily speaking, this is a strong selective penalty. Depression also appears to reduce libido and may make one unattractive as a sexual partner. Andrews and Thomson point out depression’s “adverse effect on women’s fertility and the outcome of pregnancy.“ Other health problems are comorbid with depression, although it’s not clear whether depression is the cause or consequence of these problems. Finally, studies show that depressed mothers provide poorer care of their children.

As Coyne notes, this is a problem not only for ARH, but also for a number of other evolutionary psychological accounts of depression–essentially, all those theories that posit that the depressive state itself is adaptive (as opposed to balancing selection/heterozygote advantage models which allow for the possibility that some genes that contribute to depression may be selected for under the right circumstances, without implying that depression itself is advantageous).

de Waal and Ferrari on cognition in humans and animals

Humans do many things that most animals can’t. That much no one would dispute. The more interesting and controversial question is just how many things we can do that most animals can’t, and just how many animal species can or can’t do the things we do. That question is at the center of a nice opinion piece in Trends in Cognitive Sciences by Frans de Waal and Pier Francisco Ferrari.

De Waal and Ferrari argue for what they term a bottom-up approach to human and animal cognition. The fundamental idea–which isn’t new, and in fact owes much to decades of de Waal’s own work with primates–is that most of our cognitive abilities, including many that are often characterized as uniquely human, are in fact largely continuous with abilities found in other species. De Waal and Ferrari highlight a number of putatively “special” functions like imitation and empathy that turn out to have relatively frequent primate (and in some cases non-primate) analogs. They push for a bottom-up scientific approach that seeks to characterize the basic mechanisms that complex functionality might have arisen out of, rather than (what they see as) “the overwhelming tendency outside of biology to give human cognition special treatment.”

Although I agree pretty strongly with the thesis of the paper, its scope is also, in some ways, quite limited: De Waal and Ferrari clearly believe that many complex functions depend on homologous mechanisms in both humans and non-human primates, but they don’t actually say very much about what these mechanisms might be, save for some brief allusions to relatively broad neural circuits (e.g., the oft-criticized mirror neuron system, which Ferrari played a central role in identifying and characterizing). To some extent that’s understandable given the brevity of TICS articles, but given how much de Waal has written about primate cognition, it would have been nice to see a more detailed example of the types of cognitive representations de Waal thinks underlie, say, the homologous abilities of humans and capuchin monkeys empathize with conspecifics.

Also, despite its categorization as an “Opinion” piece (these are supposed to stir up debate), I don’t think many people (at least, the kind of people who read TICS articles) are going to take issue with the basic continuity hypothesis advanced by de Waal and Ferrari. I suspect many more people would agree than disagree with the notion that most complex cognitive abilities displayed by humans share a closely intertwined evolutionary history with seemingly less sophisticated capacities displayed by primates and other mammalian species. So in that sense, de Waal and Ferrari might be accused of constructing something of a straw man. But it’s important to recognize that de Waal’s own work is a very large part of the reason why the continuity hypothesis is so widely accepted these days. So in that sense, even if you already agree with its premise, the TICS paper is worth reading simply as an elegant summary of a long-standing and important line of research.

if natural selection goes, so does most everything else

Jerry Fodor and Massimo Piattelli-Palmarini have a new book out entitled What Darwin Got Wrong. The book has—to put it gently—not been very well received (well, the creationists love it). Its central thesis is that natural selection fails as a mechanism for explaining observable differences between species, because there’s ultimately no way to conclusively determine whether a given trait was actively selected for, or if it’s just a free-rider that happened to be correlated with another trait that truly was selected for. For example, we can’t really know why polar bears are white: it could be that natural selection favored white fur because it allows the bears to blend into their surroundings better (presumably improving their hunting success), or it could be that bears with sharper teeth happen to have white fur, or that smaller, less energetic bears who need to eat less often tend to have white fur, or that a mutant population of polar bears who happened to be white also happened to have a resistance to some deadly disease that wiped out all non-white polar bears, or… you get the idea.

If this sounds like pretty silly reasoning to you, you’re not alone. Virtually all of the reviews (or at least, those written by actual scientists) have resoundingly panned Fodor and Piattelli-Palmarini for writing a book about evolution with very little apparent understanding of evolution. Since I haven’t read the book, and can’t claim much knowledge of evolutionary biology, I’m not going to weigh in with a substantive opinion, except to say that, based on the reviews I’ve read, along with an older article of Fodor’s that makes much the same argument, I don’t see any reason to disagree with the critics. The most elegant critique I’ve come across is Block and Kitcher’s review of the book in the Boston Review:

The basic problem, according to Fodor and Piattelli-Palmarini, is that the distinction between free-riders and what they ride on is “invisible to natural selection.“ Thus stated, their objection is obscure because it relies on an unfortunate metaphor, introduced by Darwin. In explaining natural selection, the Origin frequently resorts to personification: “natural selection is daily and hourly scrutinising, throughout the world, every variation, even the slightest“ (emphasis added). When they talk of distinctions that are “invisible“ to selection, they continue this personification, treating selection as if it were an observer able to choose among finely graded possibilities. Central to their case is the thesis that Darwinian evolutionary theory must suppose that natural selection can make the same finely graded discriminations available to a human (or divine?) observer.

Neither Darwin, nor any of his successors, believes in the literal scrutiny of variations. Natural selection, soberly presented, is about differential success in leaving descendants. If a variant trait (say, a long neck or reduced forelimbs) causes its bearer to have a greater number of offspring, and if the variant is heritable, then the proportion of organisms with the variant trait will increase in subsequent generations. To say that there is “selection for“ a trait is thus to make a causal claim: having the trait causes greater reproductive success.

Causal claims are of course familiar in all sorts of fields. Doctors discover that obesity causes increased risk of cardiac disease; atmospheric scientists find out that various types of pollutants cause higher rates of global warming; political scientists argue that party identification is an important cause of voting behavior. In each of these fields, the causes have correlates: that is why causation is so hard to pin down. If Fodor and Piattelli-Palmarini believe that this sort of causal talk is “conceptually flawed“ or “incoherent,“ then they have a much larger opponent then Darwinism: their critique will sweep away much empirical inquiry.

This really seems to me to get at the essence of the claim, and why it’s silly. Fodor and Piattelli-Palmarini are essentially claiming that natural selection is bunk because you can never be absolutely sure that natural selection operated on the trait you think it operated on. But scientists don’t require absolute certainty to hold certain beliefs about the way the world works; we just require that those beliefs seem somewhat more plausible than other available alternatives. If you take absolute certainty as a necessary criterion for causal inference, you can’t do any kind of science, period.

It’s not just evolutionary biology that suffers; if you held psychologists to the same standards, for example, we’d be in just as much trouble, because there’s always some potential confound that might explain away a putative relation between an experimental manipulation and a behavioral difference. If nothing else, you can always blame sampling error: you might think that giving your subjects 200 mg of caffeine was what caused them to have to go to the bathroom every fifteen minutes report decreased levels of subjective fatigue, but maybe you just happened to pick a particularly sleep-deprived control group. That’s surely no less plausible an explanation than some of the alternative accounts for the whiteness of the polar bear suggested above. But if you take this type of argument seriously, you can pretty much throw any type of causal inference (and hence, most science) out the window. So it’s hardly surprising that Fodor and Piattelli-Palmarini’s new book hasn’t received a particularly warm reception. Most of the critics are under the impression that science is a pretty valuable enterprise, and seems to work reasonably well most of the time, despite the rampant uncertainty that surrounds most causal inferences.

Lest you think there must be some subtlety to Fodor’s argument the critics have missed, or that there’s some knee-jerk defensiveness going on on the part of, well, damned near every biologist who’s cared to comment, I leave you with this gem, from a Salon interview with Fodor (via Jerry Coyne):

Creationism isn’t the only doctrine that’s heavily into post-hoc explanation. Darwinism is too. If a creature develops the capacity to spin a web, you could tell a story of why spinning a web was good in the context of evolution. That is why you should be as suspicious of Darwinism as of creationism. They have spurious consequence in common. And that should be enough to make you worry about either account.

I guess if you really believed that every story you could come up with about web-spinning was just as good as any other, and that there was no way to discriminate between them empirically (a notion Coyne debunks), this might seem reasonable. But then, you can always make up just-so stories to fit any set of facts. If you don’t allow for the fact that some stories have better evidential support than others, you indeed have no way to discriminate creationism from science. But I think it’s a sad day if Jerry Fodor–who’s made several seminal contributions to cognitive science and the philosophy of science–really believes that.