Posted by: Chris | January 20, 2011

Against a Maximalist Darwinism

The Lure has been critical of sophistical evolutionary logic in the past, so when I came across Jesse Bering’s article about the evolution of rape defenses on Slate, it raised an eyebrow, to say the least.    I was tempted to write a rebuttal myself, but fortunately PZ Meyers and Jerry Coyne have both written excellent posts taking apart the more specious aspects of Bering’s piece.  This is especially propitious because both Meyers and Coyne, in their usual posture as defenders of the faith against the great unwashed, are quite prone to never finding a Darwinian explanation they don’t like (the charge Coyne throws Bering’s way).  Bering argues that women have evolved four key rape defenses :

  1.  When threatened by sexual assault, ovulating women display a measurable increase in physical strength.
  2. Ovulating women overestimate strange males’ probability of being rapists.
  3. Ovulating women play it safe by avoiding situations that place them at increased risk of being raped.
  4. Women become more racist when they’re ovulating.

But the evidence does not back up his assertions.  From Meyers’ post:

When you examine the literature cited in these papers, you discover that different investigators get different results — the handgrip study even admits up front that there are conflicting results, with other papers finding no differences in performance across the menstrual cycle.  None test anything to do with inheritance, none try (or even can) look at the genetic basis of the behaviors they are studying. Yet somehow evolutionary psychologists conclude that “women may have been selected during human evolution to behave in ways that reduce the likelihood of conception as a consequence of rape.”Another way to look at it is that they are hypothesizing that women are more likely to behave in ways that invite physical attack and brutal abuse when they aren’t ovulating. That is a remarkable assertion. It also carries the strange implication that the consequences of rape can be measured by the likelihood of immediate fertilization, rather than by the toll of physical injury and emotional trauma, a peculiar thing for psychologists to neglect. Wouldn’t it make more sense to have a general hypothesis that people, men and women, who can avoid violence at any time in their life, are more likely to be reproductively successful and thereby pass on their genes to subsequent generations? That’s all they’re saying, essentially, and the straining to sex it up by tying globally useful behaviors to reproductive cycles is unconvincing…

Too often investigators start with the assumption that a feature absolutely must have been selected for, or it wouldn’t be there, and then contrive elaborate rationalizations for processes that could have favored its preservation in our ancestry…and the aura of plausibility is then sufficient to conclude that it must be so, even in the absence of any supporting evidence, and sometimes even in the face of contradictory evidence.

To this I would only add, even if the papers showed strong correlations over multiple populations and that the behavior was heritable, the authors still could not make the claim that this behavior specifically was selected for.  Notice all of the defenses occur during ovulation, when a woman’s hormone balance shifts abruptly.  It seems far more plausible that the effects Bering cites, rather than be tenuous (and questionably selectively beneficial) defenses against rape, are simply neutral side effects of the spike in estrogen, progesterone, etc. that causes and accompanies ovulation, a biological pathway driven to fixation solely to help prepare the womb for incipient fertilization.  Too often we focus on specific behaviors and reason that, since these behaviors are sufficiently widespread and heritable, they must have some selective benefit.  My suspicion is, for the most part, drift and pleiotropy play a much larger role.

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