Two top Johns Hopkins University psychiatrists say there is insufficient evidence to conclude people are born with heterosexual, homosexual or bisexual attractions.

"The understanding of sexual orientation as an innate, biologically fixed property of human beings — the idea that people are 'born that way' — is not supported by scientific evidence," Drs. Lawrence Mayer and Paul McHugh conclude in their new study, "Sexuality and Gender."

"While minor differences in the brain structures and brain activity between homosexual and heterosexual individuals have been identified … such neurobiological findings do not demonstrate whether [they] are innate or are the result of environmental and psychological factors."

In addition, studies comparing brain structures of transgender and cisgender individuals demonstrate "weak correlations between brain structure and cross-gender identification" and do not provide any evidence for a neurobiological basis for cross-gender identification, they say.

According to a recent estimate, about 0.6 percent of U.S. adults identify as a gender that does not correspond to their biological sex.

Mayer is a scholar-in-residence at the Maryland school's psychiatry department and Paul McHugh, a professor of psychiatry and behavioral sciences.