New Zealand political scientist James Flynn talks about the IQ trends he discovered over at Cato Unbound. To wit:
Americans made massive IQ gains on the WISC [Wechsler Intelligence Scale for Children] between 1947 and 2002 amounting to almost 18 points of Full Scale IQ. These gains ranged from only 2 points on the WISC subtest called Information to 24 points on the subtest called Similarites (what do dogs and rabbits have in common?), despite the fact that both have the cognitive complexity that makes them good measures of g.
Obviously such a rapid increase in average IQs can't be attributed to genetic factors. Flynn's essay makes some smart observations about how the interaction of genetic and environmental factors influence an individual's level of general intelligence.
Whole Flynn essay here.