106 years of "born that way": from 1899 until just last month, scientific evidence of the biological roots of sexual orientation has continued to grow.

1899

Germany's pioneering researcher Magnus Hirschfeld writes that being gay or lesbian is biologically determined.

1991

A study of twins reveals that genes and biology are markers for homosexuality. Researchers find that 52% of identical twin brothers of gay men were also gay, compared with 22% of fraternal twins and just 11% of adoptive brothers.

1991

Simon LeVay, after studying the brains of dead gay and straight men, publishes a paper in Science arguing that homosexuality is biologically based. He later expands his ideas in the 1993 book The Sexual Brain.

1993

In a major study that spurs cultural battles, researchers say they have identified a gene pattern that determines male homosexuality, A National Cancer Institute geneticist suggests that a gay gene resides in the X chromosome. which men inherit from their mothers.

1995 & 2005

A 1995 NIH study focuses on male fruit flies that began mating with each other after their body chemistry was changed, In June 2005, scientists in Vienna report that a genetically altered female fruit fly attempted to mate with another female fruit fly instead of a male, suggesting sexual attraction is genetic.

1999

A University of Texas researcher finds that the tones produced by the inner ears of lesbians are weaker and not as numerous as those produced by straight women.

2000

Researchers find further evidence linking sexual orientation with birth order. Men with same-sex attraction seem more likely to be born later than straight brothers in families with multiple male children.

2003

Psychologists measure eye-blink reactions after subjecting gay and straight men and women to loud noises. The reactions of gay and straight members of each gender are found to be significantly different and linked to an area of the brain that determines sexuality. They conclude that a person's sexual orientation is determined before birth.

2004

The same genes that make men gay may also make their female relatives more fertile. Researchers find that the moms, sisters, and maternal aunts of gay men often have significantly more children than the mean--and many of their offspring are gay or lesbian as well.

2005

A genome researcher In Chicago says he has found a group of genes, not a single "gay gene," that strongly affect whether a man is homosexual. The University of Illinois at Chicago and the NIH scientists report having looked at the genes of 456 men, each of whom had at least two gay brothers, from 146 families. The finding that a variety of genes are common to most gay men echoes other recent gene research.