By Michelle Roberts
BBC News health reporter, in Washington DC
When it comes to brain size and intelligence, bigger is not necessarily better, say scientists.
Human brains today are triple the size of those of ancient man
Although our brains are triple the size of our primitive ancestors, history suggests the growth had nothing to do with becoming smarter.
Ancient man went through two periods where brain mass increased, yet during these times toolmaking techniques did not improve, says William Calvin.
The Washington University professor says other factors must be responsible.
Size not everything
"Everybody assumes that bigger is smarter is better. That doesn't really appear to be true," Professor Calvin told the annual meeting of the American Association for the Advancement of Science (AAAS).
"Cleverness, at least for toolmaking, did not improve despite a lot of brain size increase."
From the time when Homo sapiens were walking around in Africa 200,000 years ago with a brain size of ours, little happened in terms of intellectual revelations until 150,000 years later, according to Professor Calvin.
At this time, there was a burst of creativity with the emergence of higher intellectual functions such as more complicated language construction, chains of logic, structured music and games with arbitrary rules, he said.
"So something else was driving the size gain.
"And now brain size is actually creeping down, not creeping up if anything," he added.
He said a big brain could have advantages for skills that are linked with survival, such as throwing.
"In hunting, you have to be right the first time, or dinner runs away.
"So 'bigger is better' may be true for something."