Rule #11 Male and female brains are different. • The X chromosome that males have one of and females have two of—though one acts as a backup—is a cognitive “hot spot,” carrying an unusually large percentage of genes involved in brain manufacture. • Women are genetically more complex, because the active X chromosomes in their cells are a mix of Mom’s and Dad’s. Men’s X chromosomes all come from Mom, and their Y chromosome carries less than 100 genes, compared with about 1,500 for the X chromosome. • Men’s and women’s brains are different structurally and biochemically—men have a bigger amygdala and produce serotonin faster, for example—but we don’t know if those differences have significance. • Men and women respond differently to acute stress: Women activate the left hemisphere’s amygdala and remember the emotional details. Men use the right amygdala and get the gist.

— Brain Rules by John Medina