The social psychologist Robert Zajonc has a solution to the problem that half the studies on
the relation of birth order to intelligence show first-born children to be brighter, and half give
the advantage to the later-born. He finds that
\text{later-born children profit more from the teachings of their parents when they are older; first-born}
\text{children are brighter early on since they get individual teaching from their parents}
\text{later-born children profit more from the teaching of their parents and older siblings when they are}
\text{young; first-born children gain the IQ advantage when they are older from teaching their younger}
\text{siblings}
\text{at the approximate age of 11, the IQ levels of later-born and first-born switch; the later-born becomes}
\text{brighter and the first-born loses their early advantage}
\text{those studies testing children earlier than 11 found no birth-order difference; after the age of 11, there}
\text{was a slight IQ advantage to being first-born}
