There is strong evidence for a relationship between fraternal birth order and male sexual orientation, and there has been biological research done to investigate potential biological determinants of sexual orientation in men and women. One theory is the second to fourth finger ratio (2D:4D) theory. Some studies have discovered that heterosexual women had higher 2D:4D ratios than did lesbian women but the difference was not found between heterosexual and gay men. Similarly, a study has shown that homosexual men have a sexually dimorphic nucleus in the anterior hypothalamus that is the size of females’. Twin and family studies have also found a genetic influence. Changes in sexuality Demographics General One study by Steven E. Mock and Richard P. Eibach from 2011 shows 2% of 2,560 adult participants included in National Survey of Midlife Development in the United States reported change of sexual orientation identities after a 10-year period: 0.78% of male and 1.36% of female persons that identified themselves to be heterosexuals at the beginning of the 10-year period, as well as 63.6% of lesbians, 64.7% of bisexual females, 9.52% of gay males, and 47% of bisexual males. According to the study, "this pattern was consistent with the hypothesis that heterosexuality is a more stable sexual orientation identity, perhaps because of its normative status. However, male homosexual identity, although less stable than heterosexual identity, was relatively stable compared to the other sexual minority identities". Having only adults included in the examined group, they did not find the differences in fluidity which were affected by age of the participants. However, they stated that "research on attitude stability and change suggests most change occurs in adolescence and young adulthood (Alwin & Krosnick, 1991; Krosnick & Alwin, 1989), which could explain the diminished impact of age after that point". Males versus females Research generally indicates that female sexuality is more fluid than male sexuality. In a seminal review of the sexual orientation literature, stimulated by the findings that the 1970s sexual revolution affected female sexuality more so than male sexuality, research by Baumeister et al. indicated that when compared to males, females have lower concordance between sexual attitudes and behaviors, and sociocultural factors affect female sexuality to a greater degree; it also found that personal change in sexuality is more common for females compared to males. Female sexuality (lesbian and heterosexual) changes significantly more than males on both dimensional and categorical measures of