Marriage isn't what it used to be. (Credit: Getty Images)

Just in time for Valentine's Day, a researcher at Northwestern University has revealed what grumpy old people have been telling us for years: Marriage isn't what it used to be.

That doesn't mean marriage is worse than it was before, however.

"The average marriages are struggling these days, but the best marriages are better than ever," said Eli Finkle, a professor of social psychology at Northwestern University who spent the last nine months examining literature on marriage from historical, sociological, economical and psychological perspectives.

He found that marriage has become more about mutual self-discovery and satisfaction and, in the last several hundred years, less about steering clear of starvation. But that doesn't mean there aren't other obligations, such as full time jobs and PTA meetings. As a result, husbands and wives aren't putting in the time to make their marriages meaningful.

"Marriage has a greater potential for greatness than ever before, but a larger proportion of people are failing to meet that potential," Finkle said. "It's a supply and demand issue. If what you're asking requires profound insight into each other, you'd better invest a lot of time in each other."

His advice? Manage expectations by deciding what you're willing to expect not to get from your marriage. If you can't invest as much time because of the bad economy, for instance, be comfortable overlooking certain needs temporarily.

But, if that's the case, "hold in your back pocket that the kids will eventually get older, and the economic climate will get better," Finkle said.