by Professor Linda C. Babcock in her book Women Don't Ask shows that men are eight times more likely to ask for a pay raise, suggesting that pay inequality may be partly a result of behavioral differences between the sexes. However, studies generally find that a portion of the gender pay gap remains unexplained after accounting for factors assumed to influence earnings; the unexplained portion of the wage gap is attributed to gender discrimination. Estimates of the discriminatory component of the gender pay gap vary. The OECD estimated that approximately 30% of the gender pay gap across OECD countries is because of discrimination. Australian research shows that discrimination accounts for approximately 60% of the wage differential between men and women. Studies examining the gender pay gap in the United States show that a much of the wage differential remains unexplained, after controlling for factors affecting pay. One study of college graduates found that the portion of the pay gap unexplained after all other factors are taken into account is five percent one year after graduating and 12% a decade after graduation. A study by the American Association of University Women found that women graduates in the United States are paid less than men doing the same work and majoring in the same field. Wage discrimination is theorized as contradicting the economic concept of supply and demand, which states that if a good or service (in this case, labor) is in demand and has value it will find its price in the market. If a worker offered equal value for less pay, supply and demand would indicate a greater demand for lower-paid workers. If a business hired lower-wage workers for the same work, it would lower its costs and enjoy a competitive advantage. According to supply and demand, if women offered equal value demand (and wages) should rise since they offer a better price (lower wages) for their service than men do. Research at Cornell University and elsewhere indicates that mothers in the United States are less likely to be hired than equally qualified fathers and, if hired, receive a lower salary than male applicants with children. The OECD found that "a significant impact of children on women's pay is generally found in the United Kingdom and the United States". Fathers earn $7,500 more, on average, than men without children do. There is research to suggest that the gender wage gap