KNOCKING DOWN THE ‘UNIT COHESION’ ARGUMENT…. I’ve never heard a good argument for excluding patriotic, physically-able American volunteers from serving in the Armed Forces, simply on the basis of sexual orientation. But the most common argument has to do with “unit cohesion.”

As the claim goes, servicemen and women might be uncomfortable serving alongside soldiers who are openly gay, and that discomfort means a unit may not function as it should. It’s better, the argument goes, to exclude qualified, well-trained soldiers from service in the midst of two wars than to make anti-gay soldiers feel ill at ease.

As policymakers take steps toward undoing the absurd “Don’t Ask, Don’t Tell” policy, the evidence to support a shift keeps growing.

A survey of troops who served in Iraq and Afghanistan concluded that having gay or lesbian soldiers in fighting units has no significant impact on unit cohesion or readiness.

The data raise new doubts about the underlying assumption of the congressional ban, namely that military discipline will fall apart if gays and lesbians are permitted to serve openly.

“Service members said the most important factors for unit cohesion and readiness were the quality of their officers, training, and equipment,” said Laura Miller, a military sociologist at the RAND Corporation, a private research group that has long advised the Pentagon, which conducted the study along with the University of Florida. “Serving with another service member who was gay or lesbian was not a significant factor that affected unit cohesion or readiness to fight.”

Three-quarters of the veterans surveyed said they felt “comfortable” or “very comfortable” in the presence of gays or lesbians, and nearly one in five said they already knew of a gay or lesbian member in their unit.

As a rule, reality, evidence, and common sense hasn’t played much of a role in the debate over DADT, so it’s likely that the new study won’t have much of an effect. But the RAND Corporation’s report is a reminder that it’s probably time to stop thinking about a DADT repeal as “controversial.” It’s not — most civilians are fine with letting openly gay soldiers serve, and most of the troops agree.

Time to end the policy.

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Follow Steve on Twitter @stevebenen. Steve Benen is a producer at MSNBC's The Rachel Maddow Show. He was the principal contributor to the Washington Monthly's Political Animal blog from August 2008 until January 2012.