CHICAGO — A large study of gay brothers adds to evidence that genes influence men’s chances of being homosexual, but the results aren’t strong enough to prove it.

Some scientists believe several genes might affect sexual orientation. Researchers who led the new study of nearly 800 gay brothers say their results bolster previous evidence pointing to genes on the X chromosome.

They also found evidence of influence from a gene or genes on a different chromosome. But the study doesn’t identify which of hundreds of genes located in either place might be involved.

Smaller studies seeking genetic links to homosexuality have had mixed results.

The new evidence “is not proof but it’s a pretty good indication” that genes on the two chromosomes have some influence over sexual orientation, said Dr. Alan Sanders, the lead author. He studies behavioral genetics at NorthShore University HealthSystem Research Institute in Evanston, Illinois.

Experts not involved in the study were more skeptical.

Neil Risch, a genetics expert at the University of California, San Francisco, said the data are statistically too weak to demonstrate any genetic link. Risch was involved in a smaller study that found no link between male homosexuality and chromosome X.

Dr. Robert Green, a medical geneticist at Harvard Medical School, called the new study “intriguing but not in any way conclusive.”

The work was published Monday by the journal Psychological Medicine. The National Institutes of Health paid for the research.

The researchers say they found potential links to male homosexuality in a portion of chromosome X and on chromosome 8, based on an analysis of genetic material in blood or saliva samples from participants.

Chromosome X is one of two human sex chromosomes; the other is chromosome Y, present only in men.

The study authors note that animal research suggests a gene located in one region of chromosome X may contribute to some sexual behavior; it’s one of the same regions cited in the new study.

Specific causes of homosexuality are unknown. Some scientists think social, cultural, family and biological factors are involved, while some religious groups consider it an immoral choice.

Study participant Dr. Chad Zawitz, a Chicago physician, called the research “a giant step forward” toward answering scientific questions about homosexuality and helping reduce the stigma gays often face.

Being gay “is sort of like having certain eye color or skin color — it’s just who you are,” Zawitz said. “Most heterosexuals I know didn’t choose to be heterosexual. It’s puzzling to me why people don’t understand.”

But believing this study is a choice

Checkmate

I can't get over the sheer irony of this.