Evolution and imperfect mediators

  • Skoyles, J. R. (1999). Human evolution expanded brains to increase expertise capacity, not IQ. Psycoloquy, 10(002). Chicago

Skoyles is arguing a rather implausible claim:

Why do modern humans have larger brains than earlier people such as Homo erectus? As large brains cause problems in childbirth, infancy and locomotion, the advantage they offer must be substantial. This advantage might be associated with increased IQ, but there is a problem: evidence from MRI volumetric surveys, microcephaly and hemispherectomy shows that there exist individuals with psychometrically normal IQ but Homo-erectus-sized brains. Why did evolution increase brain size (with its associated costs) when humans (as these individuals demonstrate) can have normal IQ without bigger brains? I propose that the advantage may be related to increased capacity for an aspect of intelligent behaviour not measured by IQ tests but critical to the survival of our simple hunter-gatherers ancestors: the capacity to develop expertise.

His argument:

2. First, we should note that brain expansion beyond that of Homo erectus size causes neonatal, obstetric and female locomotor handicaps (reviewed below). Thus, whatever selected for increased brain size must have offered a very strong compensating benefit. Second, clinical evidence indicates that modern people can have brains no larger than Homo erectus yet exhibit normal IQ scores. Thus, the compensating benefit offered by large brains is unlikely to be intelligence as measured by IQ: Why should evolution have increased brain size with all its associated problems for something that Homo erectus sized brains could have without expansion?

Do you see the fallacy? It is a subtle variant of a failure to think in statistical terms. Whenever I see these kinds of arguments, I immediately try some analogous traits. A very common practice in analytic philosophy (for those that don’t know, I did spent a bunch of years doing that kind of thing).

If we condense the argument, it becomes a little clearer:

Brain expansion causes problems. Thus, whatever selected for increased brain size must have offered compensating benefits. People can have below average size brains yet exhibit normal intelligence. Thus, the compensating benefit offered by large brains is unlikely to be intelligence. Why should evolution have increased brain size with its associated problems for something smaller sized brains could have without expansion?

I merely edited out the unnecessary parts. Now try substituting some other trait, say fighting ability and some mediator of it.

Muscle size increases causes problems. Thus, whatever selected for increased muscle size must have offered compensating benefits. People can have below average size muscles yet exhibit normal fighting ability. Thus, the compensating benefit offered by large muscles is unlikely to be fighting ability. Why should evolution have increased muscle size with its associated problems for something smaller sized muscles could have without increase?

See the issue? This argument works for any imperfect physical underpinning of a trait, which is to say, basically all of them. Longer legs didn’t evolve for running well for some people with short legs run well. Bigger/stronger hears didn’t evolve for better cardio, because some people smaller/weaker hearts have good cardio. Longer arms didn’t evolve for fighting because some short armed people fight well. Darker skin didn’t evolve as a protection against sun exposure for some relative light skinned people don’t get skin cancer or sunburns. Larger eyes didn’t evolve for seeing better for some people with smaller eyes see well. Bigger ears… Bigger noses… Stronger hands… …

Views All Time
Views All Time
355
Views Today
Views Today
1