04 July 2011

Can evolutionary psychology walk the walk over walking?

ResearchBlogging.orgRobert Kurzban recently defended evolutionary psychology by asking the rhetorical question:

If you think that we can discover the function of fever, puking, and pooping at the base of a tree, why are other patterns of behavior not susceptible to a similar analysis?

You can. But there are too many examples of evolutionary biology being applied to human behaviour in an offhanded and slipshod way. For an example, John Bradshaw described this research on the detection of sex from movement. This subject was reviewed by Pollick and colleagues in 2005 here:

If you attach small lights to people’s major joints, and film them in the dark so that only the lights are visible, the latter do not appear to constitute a meaningful pattern, such as a person, at least until the individuals commence to move. If they walk, sit down, stand up, or act to throw something or to pick it up, the separate and apparently randomly distributed lights immediately seem to assemble and constitute someone performing a meaningful and recognisable activity. I wondered whether we could interpret such biological motion, as it is called, as being generated by a male or a female walker, whom we filmed on a treadmill. Trying it with some colleagues, I was at first very disappointed; we all felt that our forced choices, ‘male’ or ‘female’ for the various walkers, were entirely random. Yet later on analysis, it seemed that our scores were very often correct, well above chance.

His interpretation of this baffled me, however.

Presumably such a discriminative capacity is of major evolutionary or adaptive significance.

Wait, wait, wait... what? How on Earth does that follow?

A couple of the basic requirements for something to be adaptive is that it provide a selective advantage to those individuals (and their offspring!) that have it, and that the feature be heritable.

It would be easy to spin out a few hypotheses for what would be the selective advantage of detecting sex from a few points of lights on the body (which is extremely unlikely to be something our ancestors on the African savanna would have had to cope with). “Our primitive ancestors needed to distinguish under poor lighting conditions whether some unknown individual was a male or a female, because males might have been a greater threat than females.” That’s just a first pass, so we’ll let it slide despite any shortcomings it might have.

But not only is there no evidence that this ability is heritable, there is also an easy alternative hypothesis: learning. In human society, how many hours do you think you has spent seeing men and women walking? And in that time, it is possible that you learned some of the differences in gait between males and females that allows you to tell them apart.

I know that a radio editorial is not a peer-reviewed journal. I know I’m picking on a single throw-away line, not the major point of the piece. But I think it is still an excellent example of how quick people are to jump to evolutionary explanations for human capabilities, completely bypassing other explanations.

As Stephen Jay Gould and Dick Lewontin argued so famously, not everything is adaptive. Some things are by-products, or, as they termed them, spandrels. Their criticisms are as still relevant now, especially for evolutionary psychology.


Gould SJ, Lewontin RC. 1979. The spandrels of San Marco and the Panglossian paradigm: A critique of the adaptationist programme. Proceedings of the Royal Society B: Biological Sciences 205(1161): 581-598. DOI: 10.1098/rspb.1979.0086

Pollick F, Kay J, Heim K, Stringer R. 2005. Gender recognition from point-light walkers. Journal of Experimental Psychology: Human Perception and Performance 31(6): 1247-1265. DOI: 10.1037/0096-1523.31.6.1247

Photo by danceinthesky on Flickr; used under a Creative Commons license.


Andrew Wilson said...

Point light displays are a method for examining what we can perceive from structured motion alone. It turns out we can perceive quite a lot from motion, including the gender and emotional state of a person.

The suggestion is that biological motion perception (not the perception of point lights) is what actually underpins these abilities in more naturalistic conditions.

So while your point about being careful about claiming adaptationism is fair, the way you've addressed this particular topic isn't fair to the research. Our ability to directly access all kinds of very important information about animate organisms in our surrounds via motion perception is, in fact, extremely adaptive. The fact that this is measured in the lab using point lights is not even approximately the issue.

I recommend checking out Niko Troje's work at http://www.biomotionlab.ca/. He's one of the best I know in the field.

Zen said...

Thanks for the comment. I'm particularly please you thought the warning about adaptation was fair, as it was the main one I was trying to make.

Whether there are adaptations for detecting biological motion above and beyond those for detecting motion in general is an interesting question. For humans in particular, I still think the key point is trying to rule out that detection of biological motion is different than plain old learning.

Andrew Wilson said...

For humans in particular, I still think the key point is trying to rule out that detection of biological motion is different than plain old learning.
That distinction doesn't make sense. The thing you are learning is what the biological motion specifies. You don't learn about gender, and learn about biological motion, and learn that one can be used to perceive the other. The claim is that you distinguish gender on the basis of the information in biological motion.

And actually, while the warning about adaptationism is fair, the reason I felt moved to comment is that I don't think it's justified in the context of this study.

Zen said...

Er. I don't quite understand.

What feature (or ability, I suppose) is an adaptation, in your view?

Is the claim that "detecting biological motion" is an adaptation?

Looking at it again, I do see how Bradshaw might have been referring to the more general case of biological motion in the quote, rather than detection of sex specifically, which was obviously my initial interpretation.

Andrew Wilson said...

It certainly reads that way to me, and no-one studying biological motion perception would claim it was gender perception that was the key thing (I don't think; there are crazy people everywhere, of course :)

But the detection of biological and other complex motion (and, more specifically, the perception of what that complex motion specifies out in the world) is utterly critical to successful visual perception; these complex motions are the only possible source of information good enough to support perception and action. I don't think it's controversial to say that it's what the visual system has evolved to detect.