circumstantial evidence. Of course, tomorrow afternoon genetic mechanisms producing racial and ethnic differences in intelligence might be discovered, but there have been a lot of investigations, and tomorrow has not come for quite some time now." concurred, noting that while several environmental factors have been shown to influence the IQ gap, the evidence for a genetic influence has been negligible. A 2012 review by concluded that the entire IQ gap can be explained by known environmental factors, and Mackintosh found this view to be plausible. More recent research attempting to identify genetic loci associated with individual-level differences in IQ has yielded promising results, which led the editorial board of Nature to issue a statement differentiating this research from the "racist" pseudoscience which it acknowledged has dogged intelligence research since its inception. It characterized the idea of genetically determined differences in intelligence between races as definitively false. Heritability within and between groups Twin studies of intelligence have reported high heritability values. However, these studies have been criticized for being based on questionable assumptions. When used in the context of human behavior genetics, the term "heritability" can be misleading, as it does not necessarily convey information about the relative importance of genetic or environmental factors on the development of a given trait, nor does it convey the extent to which that trait is genetically determined. Arguments in support of a genetic explanation of racial differences in IQ are sometimes fallacious. For instance, hereditarians have sometimes cited the failure of known environmental factors to account for such differences, or the high heritability of intelligence within races, as evidence that racial differences in IQ are genetic. Psychometricians have found that intelligence is substantially heritable within populations, with 30–50% of variance in IQ scores in early childhood being attributable to genetic factors in analyzed US populations, increasing to 75–80% by late adolescence. In biology heritability is defined as the ratio of variation attributable to genetic differences in an observable trait to the trait's total observable variation. The heritability of a trait describes the proportion of variation in the trait that is attributable to genetic factors within a particular population. A heritability of 1 indicates that variation correlates fully with genetic variation and a heritability of 0 indicates that there is no correlation between the trait and genes at all. In psychological testing, heritability tends to be understood as the degree of correlation between the results