environment Different aspects of the socioeconomic environment in which children are raised have been shown to correlate with part of the IQ gap, but they do not account for the entire gap. According to a 2006 review, these factors account for slightly less than half of one standard deviation. Other research has focused on different causes of variation within low socioeconomic status (SES) and high SES groups. In the US, among low SES groups, genetic differences account for a smaller proportion of the variance in IQ than among high SES populations. Such effects are predicted by the bioecological hypothesis—that genotypes are transformed into phenotypes through nonadditive synergistic effects of the environment. suggest that high SES individuals are more likely to be able to develop their full biological potential, whereas low SES individuals are likely to be hindered in their development by adverse environmental conditions. The same review also points out that adoption studies generally are biased towards including only high and high middle SES adoptive families, meaning that they will tend to overestimate average genetic effects. They also note that studies of adoption from lower-class homes to middle-class homes have shown that such children experience a 12 to 18 point gain in IQ relative to children who remain in low SES homes. A 2015 study found that environmental factors (namely, family income, maternal education, maternal verbal ability/knowledge, learning materials in the home, parenting factors, child birth order, and child birth weight) accounted for the black–white gap in cognitive ability test scores. Test bias A number of studies have reached the conclusion that IQ tests may be biased against certain groups. The validity and reliability of IQ scores obtained from outside the United States and Europe have been questioned, in part because of the inherent difficulty of comparing IQ scores between cultures. Several researchers have argued that cultural differences limit the appropriateness of standard IQ tests in non-industrialized communities. A 1996 report by the American Psychological Association states that intelligence can be difficult to compare across cultures, and notes that differing familiarity with test materials can produce substantial differences in test results; it also says that tests are accurate predictors of future achievement for black and white Americans, and are in that sense unbiased. The view that tests accurately predict future educational attainment is reinforced by Nicholas Mackintosh in his 1998 book IQ and Human Intelligence, and by a 1999 literature