Amid the legal arguments at Tuesday's Supreme Court hearing on same-sex marriage, there loomed a social science question: How well do children turn out when they are raised by gay parents?

Justice Anthony Kennedy, who is widely considered the swing vote, called the topic "uncharted waters."

Indeed, gay marriage is a relatively new phenomenon in the United States. It has only been legal since 2004, when Massachusetts began issuing marriage licenses to same-sex couples. Eight more states and Washington have legalized same-sex nuptials since then, but it is has been banned in 35 states.