How to know you're in a long-term relationship, according to science

It also changes you.

More importantly, close relationships may spark an entirely different way of thinking and acting, something Joshua Wolf Shenk, author of "Powers of Two," chalks up to having a "shared mind." Shenk, who has written extensively about psychiatry and psychology for outlets including The Atlantic and The New York Times, also directs the Erikson Prize for Mental Health Media.

So, how do you and your significant other stack up? Check out these signs psychologists have observed in long-term couples that they say point to having such a shared mind.