According to palaeontologists (scientists who study fossils), over the last 20,000 years the average volume has been decreasing – possibly losing as much as 150cc (a chunk the size of a tennis ball). One possible explanation is related to the fact that brain size is correlated with body size. Humans have become smaller over the millennia. Early humans were much brawnier for hunting and also for dealing with cold climates, but now we are smaller and therefore our brains have become correspondingly smaller.
Another possibility is that brain structure has become more efficient so that fewer cells and connections are needed. There is certainly evidence that brain size does not equate to intelligence and that what may be more significant is the organisation of the brain (see A2 Complete Companion page 131). This is the suggestion made by John Hawks, who argues that the brain consumes a lot of energy therefore individuals with intelligence and a smaller brain would be selected for.
On the other hand, cognitive psychologist David Geary proposes that our brains are getting smaller because we are becoming more stupid. The argument goes that brain size is related to social complexity but in a surprising way. When population sizes were small brain size did increase as social groups got bigger but only up to a certain level. When human population groups became very large, brain size (relative to body size) started decreasing. Geary points to the fact that brain size started decreasing (about 10-15,000 years ago) at the same time as complex societies emerged. When this happened people didn’t need to be as smart to stay alive because they could rely on others for different kinds of help.
Yet another possibility is decreased aggressiveness. Richard Wrangham, a primatologist, points out that as animals become domesticated there is a reduction in their brain size. In one study where animals were selected for lack of aggressiveness, brain size reduced. Wrangham argues that this has been happening in human societies where high levels of aggressiveness are punished by death or social exclusion.
You can read more about these explanations here.