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WASHINGTON - Gay men's brains respond differently from those of heterosexual males when exposed to a sexual stimulus, researchers have found. The homosexual men's brains responded more like those of women when the men sniffed a chemical from the male hormone testosterone. "It is one more piece of evidence ... that is showing that sexual orientation is not all learned," said Sandra Witelson, an expert on brain anatomy and sexual orientation at the Michael G. DeGroote School of Medicine at McMaster University in Ontario, Canada.