Theory: The stress and pain of the first year of raising a child — low sleep, long irrational crying bouts, that sort of thing —' bring about this state in parents. Children, exposed to their parents' true, stressful, selves in the first year, learn their most basic behaviors in this time, how to react to stress. This time therefore passes on more information from parent to child than a less traumatic parenting experience would.
Could the parental stress of the first year of raising a child give the species an advantage over others, in terms of long-term intelligence, due to the ability to pass on more information from generation to generation?
There's something there. I bet someone's written or researched on it, but... maybe not.