between one generation and the next, it is highly unlikely that genetic factors could have accounted for the increasing scores, which must then have been caused by environmental factors. The importance of the Flynn effect in the debate over the causes of the black/white IQ gap lies in demonstrating that environmental factors may cause changes in test scores on the scale of 1 standard deviation. This had previously been doubted. A separate phenomenon from the Flynn effect has been the discovery that the IQ gap was gradually closing over the last decades of the 20th century, as black test-takers increased their average scores relative to white test-takers. For instance, Vincent reported in 1991 that the black–white IQ gap was decreasing among children, but that it was remaining constant among adults. Similarly, a 2006 study by Dickens and Flynn estimated that the difference between mean scores of black people and white people closed by about 5 or 6 IQ points between 1972 and 2002, a reduction of about one-third. In the same period, the educational achievement disparity also diminished. Reviews by Flynn and Dickens, Mackintosh, and Nisbett et al. accept the gradual closing of the gap as a fact. Environmental influences on group differences in IQ Health and nutrition Environmental factors including childhood lead exposure, low rates of breast feeding, and poor nutrition are significantly correlated with poor cognitive development and functioning. For example, childhood exposure to associated with homes in poorer correlates with an average IQ drop of 7 points, and iodine deficiency causes a decline, on average, of 12 IQ points. Such impairments may sometimes be permanent, but in some cases they be partially or wholly compensated for by later growth. The first two years of life are critical for malnutrition, the consequences of which are often irreversible and include poor cognitive development, educability, and future economic productivity. The African American population of the United States is statistically more likely to be exposed to many detrimental environmental factors such as poorer neighborhoods (noise, crime, limited outdoor space, and other impoverishments), air pollution (diesel exhaust, smoke, industrial emissions, ultrafine particulates, etc.), subpar schools (insufficient funding, neglect, and disruption of schooling for myriad reasons), malnutrition (high levels of nutritionally bankrupt processed foods), and subpar prenatal and postnatal health care (inadequate funding). Mackintosh points out that, for American black people, infant mortality is about twice as high as for white people, and