More General Social Study data: A reader, perhaps inspired by Inductivist and Half Sigma, has fallen in love with the GSS database of in-depth interviews of thousands of Americans. The GSS includes a 10 question vocabulary test, which is often used as a crude approximation of IQ. So, he sent me the average age at first birth by mothers in the sample by number of vocabulary words correct (0 to 10). I worked out approximate IQs for the mean member of each group and whipped together a graph in Excel. So, age at first birth is fairly flat for the left half of the bell curve (the blue line), then rises steadily for the right half. So, smarter women not only are having fewer babies (I assume), but they are starting later in life so their generation times are longer as well.