Intelligence.

Some people are cleverer than others. I think it would be a good thing if more biologists began with that observation as the starting point for their research. Why? Because it is a prominent and consistent way in which people differ from each other; because the measurements we make of people's cleverness produce scores that are correlated with important life outcomes; because it is interesting to discover the mechanisms that produce these individual differences; and because understanding these mechanisms might help to ameliorate those states in which cognitive function is low or declining.