<P> A 2005 study presented data supporting the nutrition hypothesis, which predicts that gains will occur predominantly at the low end of the IQ distribution, where nutritional deprivation is probably most severe . An alternative interpretation of skewed IQ gains could be that improved education has been particularly important for this group . Richard Lynn makes the case for nutrition, arguing that cultural factors cannot typically explain the Flynn effect because its gains are observed even at infant and preschool levels, with rates of IQ test score increase about equal to those of school students and adults . Lynn states that "This rules out improvements in education, greater test sophistication, etc. and most of the other factors that have been proposed to explain the Flynn effect . He proposes that the most probable factor has been improvements in pre-natal and early post-natal nutrition ." </P> <P> A century ago, nutritional deficiencies may have limited body and organ functionality, including skull volume . The first two years of life is a critical time for nutrition . The consequences of malnutrition can be irreversible and may include poor cognitive development, educability, and future economic productivity . On the other hand, Flynn has pointed to 20 - point gains on Dutch military (Raven's type) IQ tests between 1952, 1962, 1972, and 1982 . He observes that the Dutch 18 - year - olds of 1962 had a major nutritional handicap . They were either in the womb, or were recently born, during the great Dutch famine of 1944--when German troops monopolized food and 18,000 people died of starvation . Yet, concludes Flynn, "they do not show up even as a blip in the pattern of Dutch IQ gains . It is as if the famine had never occurred ." It appears that the effects of diet are gradual, taking effect over decades (affecting mother as well as child) rather than a few months . </P> <P> In support of the nutritional hypothesis, it is known that, in the United States, the average height before 1900 was about 10 cm (∼ 4 inches) shorter than it is today . Possibly related to the Flynn effect is a similar change of skull size and shape during the last 150 years . Though the idea that brain size is unrelated to race and intelligence was popularized in the 1980s, studies continue to show significant correlations . (Note, however, that size isn't everything--or maybe anything significant at all . For example, the overall size and asymmetrical shape of Albert Einstein's brain were normal; whereas the prefrontal, somatosensory, primary motor, parietal, temporal and occipital cortices were extraordinary .) A Norwegian study found that height gains were strongly correlated with intelligence gains until the cessation of height gains in military conscript cohorts towards the end of the 1980s . Both height and skull size increases probably result from a combination of phenotypic plasticity and genetic selection over this period . With only five or six human generations in 150 years, time for natural selection has been very limited, suggesting that increased skeletal size resulting from changes in population phenotypes is more likely than recent genetic evolution . </P> <P> It is well known that micronutrient deficiencies change the development of intelligence . For instance, one study has found that iodine deficiency causes a fall, on average, of 12 IQ points in China . </P>

The flynn effect demonstrates that later generations are smarter than previous generations