groups. in a study with han, berber, uygur and jamaican children as subjects, manning et al. found that han children had the highest mean values of 2d:4d (0.954±0.032), they were followed by the berbers (0.950±0.033), then the uygurs (0.946±0.037), and the jamaican children had the lowest mean 2d:4d (0.935±0.035). this variation is far larger than the differences between sexes; in manning's words, "there's more difference between a pole and a finn, than a man and a woman." the standard deviations associated with each given 2d:4d mean are considerable. for example, the ratio for han children (0.954±0.032) allows for a ratio as low as 0.922, while the ratio for jamaican children (0.935±0.035) allows for a ratio as high as 0.970. thus, some ethnic groups' confidence intervals overlap. a 2008 study by lu et al. found that the mean values of 2d:4d of the hui and the han in ningxia were lower than those in european countries like britain. in 2007 manning et al. also found that mean 2d:4d varied across ethnic groups with higher ratios for whites, non-chinese asians, and mid-easterners and lower ratios in chinese and black samples. two studies explored the question of whether geographical differences in 2d:4d ratios were caused by gene pool differences or whether some environmental variable associated with latitude might be involved (e.g., exposure to sunlight or different day-length patterns). the conclusions were that geographical differences in 2d:4d ratio were caused by genetic pool differences, not by geographical latitude. consanguinous parentage (inbreeding) has been found to lower the 2d:4d ratio in offspring, which may account for some of the geographical and ethnic variation in 2d:4d ratios, as consanguinity rates depend on, among others, religion, culture, and geography. digit ratio and development there is some evidence that 2d:4d ratio may also be indicative for human development and growth. ronalds et al. (2002) showed that men who had an above average placental weight and a shorter neonatal crown-heel length had higher 2d:4d ratios in adult life. kumar et al. have reported that a similar effect of hand preference on digit lengths and digit ratios occurs oppositely among children and adults. moreover, studies about 2d:4d correlations with face shape suggest that testosterone exposure early in life may set some constraints for subsequent development. prenatal sex steroid ratios (in terms of 2d:4d) and actual chromosomal sex dimorphism were found to operate differently on human faces, but affect male and female face shape by similar patterns. fink et al. (2004) found that men with low (indicating high testosterone) and women with high (indicating high estrogen) 2d:4d ratios express greater levels of facial symmetry. criticism the digit ratio has received attention within mainstream media because of supposed correlations with health and behavior, however the credibility and usefulness of research into the relationship between digit ratios and traits has been subject to criticism. critics have found research to be lacking moderate to strong statistical significance, consistently failing to be replicated, useless as a proxy variable, and have compared the research to pseudoscience. although research into digit ratios appears to find statistical significance, there has been criticism of researchers for failing to account for a majority of the variation within a dataset, with one of the most highly publicized papers on the topic failing to account for 96% of variation. furthermore, skepticism has also been attributed to an abundance of research lacking a cause-and-effect relationship between digit ratio and supposed traits, research that often lacks any theoretical motivation. this criticism was demonstrated in the paper "giving science the finger—is the second-to-fourth digit ratio (2d:4d) a biomarker of good luck? a cross sectional study" by correlating low 2d:4d digit ratio with good luck, despite following established research methods. the utilization of digit ratio as a proxy variable has come under scrutiny, as studies that investigate the relationship between a trait and some unobservable variable fail to consider other possible confounding variables. this is especially problematic in cases where the relationship is likely to be weak, such as prenatal testosterone. furthermore, critics have pointed out that controlling for confounding variables is an impossible task due to the expanding list of confounding variables, including ethnicity, sex, and physical, medical or behavioral conditions. researchers have also pointed out that research into the digit ratio is illustrative of the ongoing replication crisis. not only have subsequent studies and meta-analysis failed to replicate previous research, but have often found contradictory results, or failed closer scrutiny. psychologist martin voracek has criticized the field for its irreproducible findings, stating it's "like a house of cards built on an unknown and uncertain base" and subsequently compared "the work on finger ratios to phrenology or physiognomy, the discredited ideas that people's head shape or facial features, respectively, reveal their personalities, character, and intelligence." other animals dennis mcfadden and collaborators have demonstrated sexual dimorphism in hind limb digit ratio in a number of great apes, including gorillas and chimpanzees. emma nelson and susanne shultz are currently investigating how 2d:4d relates to primate mating strategies and the evolution of human sociality. nancy burley's research group has demonstrated sexual dimorphism in zebra finches, and found a correlation between digit ratio in females and the strength of their preference for sexually selected traits in males. alžbeta talarovičová and collaborators found in rats that elevated testosterone during the prenatal period can influence 4d length, the 2d:4d ratio, and open field motor activity. peter l. hurd, theodore garland jr., and their students have examined hindlimb 2d:4d in lines of mice selectively bred for high voluntary wheel-running behavior (see experimental evolution). these high-runner mice exhibit increased 2d:4d. this apparent "feminization" is opposite to the relation seen between 2d:4d and physical fitness in human beings, and is difficult to reconcile with the idea that 2d:4d is a clear proxy for prenatal androgen exposure in mice. the authors suggest that 2d:4d may more accurately reflect effect of glucocorticoids or other factors that regulate any of various genes. in pheasants, the ratio of the 2nd to 4th digit of the foot has been shown to be influenced by manipulations of testosterone in the egg. studies in mice indicate that prenatal androgen acts primarily by promoting growth of the fourth digit.