Why do some medical studies exclude gays, lesbians?

Perhaps an institutional bias from the historically conservative medical establishment? Or maybe some researchers believe gay men do not suffer from erectile dysfunction? A few years ago there was controversy over the blanket exclusion of minority's and women from many studies, for instance heart medications.
D.H.
MILWAUKEE - A small but significant portion of medical studies exclude gays from participating, sometimes without an apparent scientific reason, several cancer researchers say.
In a letter in Thursday's New England Journal of Medicine, three scientists from the Fox Chase Cancer Center in Philadelphia cite several dozen studies requiring a participant to be "in a reciprocal relationship with a person of the opposite sex."
There are legitimate scientific reasons for excluding gays from certain studies. Scientists would want only heterosexuals if they were studying how HIV spreads during male-female sex, for example.
But the Fox Chase folks found cases where the reason for excluding gays is not clear: tests of a drug for attention-deficit disorder, a treatment for erection problems after prostate cancer surgery, and studies on sexual function related to diabetes, depression and benign enlargement of the prostate as men age.
Brian Egleston, a biostatistician at Fox Chase, made the observation while overseeing enrolment of patients into clinical trials at the cancer centre.
"When I first saw this, I thought it was a fluke. The second time, I thought I'd dig deeper," he said.
Egleston and Roland Dunbrack Jr., a biologist, and Dr. Michael J. Hall, a medical oncologist, did a spot check of a government database of thousands of studies and turned up more examples, most of them private-industry trials.
Researchers seeking federal money for their work must explain why a study excludes a group based on gender, race or ethnicity, but no explanation is needed for exclusion based on sexual orientation, Egleston said.
Exclusion can become self-perpetuating: Researchers designing a study often "cut and paste" participation criteria from earlier trials on a similar subject.
"It becomes the way it's done," and any bias gets repeated, Egleston said.
Estimates of how much of the U.S. population is gay or bisexual vary widely; some polls have put it around 4 per cent.
Source CBC News
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Medical journal: http://www.nejm.org