Christopher Stringer, a paleoanthropologist and research leader on human origins at the Natural History Museum in London, replies:
Indeed, skeletal evidence from every inhabited continent suggests that our brains have become smaller in the past 10,000 to 20,000 years. How can we account for this seemingly scary statistic?
Some of the shrinkage is very likely related to the decline in humans' average body size during the past 10,000 years. Brain size is scaled to body size because a larger body requires a larger nervous system to service it. As bodies became smaller, so did brains. A smaller body also suggests a smaller pelvic size in females, so selection would have favored the delivery of smaller-headed babies.
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What explains our shrinking body size, though? This decline is possibly related to warmer conditions on the earth in the 10,000 years after the last ice age ended. Colder conditions favor bulkier bodies because they conserve heat better. As we have acclimated to warmer temperatures, the way we live has also generally become less physically demanding, which overall serves to drive down body weights.
Another likely reason for this decline is that brains are energetically expensive and will not be maintained at larger sizes unless it is necessary. The fact that we increasingly store and process information externally—in books, computers and online—means that many of us can probably get by with smaller brains. Some anthropologists have also proposed that larger brains may be less efficient at certain tasks, such as rapid computation, because of longer connection pathways.
The way we live may have affected brain size. For instance, domesticated animals have smaller brains than their wild counterparts probably because they do not require the extra brainpower that could help them evade predators or hunt for food. Similarly, humans have become more domesticated. But as long as we keep our brains fit for our particular lifestyles, there should be no reason to fear for the collective intelligence of our species.