· based on data from 20 countries:
· average intelligence is set at (defined as) 100. The scoring of tests has to be altered on a regular basis to maintain this (similar to GRE scores etc.)

mean IQ: · in US up 24 points since 1918
· in Netherlands: up 20 points in 30 years

This is referred to as the Flynn effect, after James Flynn, a New Zealand political scientist interested in refuting the racists views of intelligence

 The typical Englishman 100 years ago had an IQ that would have led him to be labeled (by those who use such labels) "mentally retarded," but - there has been no obvious explosion of geniuses 

Nutrition? Not likely as basic needs met long ago (but Netherlands?)
Education? Probably - more people educated for longer
Kiddie culture? - educational toys etc, mazes, blocks etc. -- highest rise is in visuo-spatial components of IQ tests. E.g., whereas first exposure to a maze might be on an IQ test 50 years ago, not kids see them everywhere -- place mats in fast-food chains etc. (Other studies have shown Tetris™ players are better at puzzle games and tests.) 

Perhaps some are not increasing but with computers, video games etc., visual-spatial skills are.
Some historical categorizations of intelligence:
Spearman: 1927: 2 factors: g - general mental ability, s - specific to particular test
later: Cattell and Horn (1962, '67, '82): 2 factors: 1) Fluid intelligence - abstract problem solving: not taught and thought to be free of experiential effects, 2) Crystallized intelligence - dependent upon knowledge and schooling