Will women be as good at killing as men? Will Marines fire teams become less tough if a woman is alongside? How about co-ed Navy SEALS going after Bin Laden-style terrorists?

These arguments against putting women in harm's way were swept away Thursday as Secretary of Defense Leon Panetta announced he is a opening the doors to women to serve in 230,000 frontline combat positions, as well as possibly elite special forces units.

It was a decision supported unanimously by the Joints Chiefs of Staff.

"Every person in today's military has made a solemn commitment to fight, and if necessary to die, for our nation's defense," Panetta said. "We owe it to them to allow them to pursue every avenue of military service for which they are fully prepared and qualified. Their career success and their specific opportunities should be based solely on their ability to successfully carry out an assigned mission. Everyone deserves that chance."

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But some critics say Panetta's decision will only cause trouble for troops abroad. Aside from size and strength differences, some say women don't have what it takes to deal with the psychological stresses of combat.

"There are substantial groups of differences between women and men in physical risk-taking and the willingness to kill strangers," said Kingsley Browne, a law professor at Wayne State University and author of the 2007 book "Coed Combat: New Evidence Shouldn't Fight the Nation's Wars."

"There is some overlap between the sexes, so there are some women who do have those characteristics, but you don't know ahead of time who is going to end up doing well or poorly in battle," Browne said.

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Browne said a problem could occur with men who feel they have to protect women during a firefight or other dangerous situations, and that men in the same unit may compete for the attention of a female, whether or not she's interested. Browne said the Israeli Defense Forces used women in combat roles in the early days of their nation, but since then have reduced their role because men wanted to protect them.

In recent years, however, Canada and Australia have opened all close arms combat jobs to women – although their numbers are few.

In Afghanistan and Iraq, the U.S. Marines Corps have deployed women in special Female Engagement Teams from 2009 to late 2012 to search local women for weapons, interview them and perform other tasks men are prohibited from performing in Islamic culture. These FETs have come under and returned fire, notes Nancy Duff Campbell, co-president of the National Women's Law Center in Washington.

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"Can women kill? Women have been in combat for a long time, sometimes unofficially," Campbell said. "Women are flying combat aircraft and ships where they have to kill, serving as gunners in tanks since the early 1990s. There doesn't seem to be any evidence that women are less likely to pull the trigger than men."

Campbell said the decision to lift the combat ban for women was done after consulting generals on the policy.

"Mostly from what I hear from military men is if (women) can do it, fine, but you shouldn't lower the standards."