A new article uses full-distribution data (as opposed to a simple comparison of means) to determine that IQ gains across thirty years (also known as the "Flynn effect") are due to better nutrition of the less privileged classes. There has been no substantial increase at the higher-IQ end of the distribution, unlike the less privileged classes which are "catching up" in terms of IQ.
Interestingly, better nutrition among the less privileged has served to reduce the spread of IQ which has a lower standard deviation in the more recent sample than in the one taken 30 years earlier. We know that the standard deviation of the IQ distribution shows both great variation in different populations and geographical structuring. Therefore, it appears that populations with the same mean IQ may have a different potential for enhancing their IQ via better nutrition in the future.
Intelligence (Article in press)
The generational intelligence gains are caused by decreasing variance in the lower half of the distribution: Supporting evidence for the nutrition hypothesis
Roberto Colom et al.
Generational intelligence gains are one intriguing finding in science. Nutrition and cognitive stimulation are among the most remarkable causes of the upward trend in intelligence. The nutrition hypothesis predicts a primary impact on the most deprived, producing disproportionate gains at low intelligence levels. The cognitive stimulation hypothesis predicts gains along the intelligence distribution. However, data from the entire distribution are rarely available. The present study compares a sample of children tested in 1970 with an equivalent sample tested 30 years later. Data for the entire distributions were available. The results are consistent with the nutrition hypothesis, because the gains were mainly concentrated in the lower and medium halves of the distribution and were negligible in the very top half of the distribution. Moreover, an impressive gradual decrease in the gains was observed from the lower half to the top half of the distribution.