The effect becomes stronger with each successive male pregnancy, meaning the odds of the next son being gay increase by 38–48%. This does not mean that all or most sons will be gay after several male pregnancies, but rather, the odds of having a gay son increase from approximately 2% for the first born son, to 4% for the second, 6% for the third and so on. Scientists have estimated between 15% and 29% of gay men may owe their sexual orientation to this effect, but the number may be higher, as prior miscarriages and terminations of male pregnancies may have exposed their mothers to Y-linked antigens. The fraternal birth order effect would not likely apply to first born gay sons; instead, scientists say they may owe their orientation to genes, prenatal hormones and other maternal immune responses which also influence brain development. This effect is nullified if the man is left-handed. Ray Blanchard and Anthony Bogaert are credited with discovering the effect in the 1990s, and Blanchard describes it as "one of the most reliable epidemiological variables ever identified in the study of sexual orientation". J. Michael Bailey and Jacques Balthazart say the FBO effect demonstrates that sexual orientation is heavily influenced by prenatal biological mechanisms rather than unidentified factors in socialization.