Brain size predicts about 20 percent of the variance in her IQ scores (her prefrontal cortex, just behind her forehead, is particularly prescient). Brain volume is related to birth weight, which means that, to a point, larger babies are smarter babies. The increase slows as baby reaches 6.5 pounds: There is only 1 IQ point difference between a 6.5-pounder and a 7.5-pounder.
