Dr. Penny Gordon-Larsen

According to Gordon-Larsen, when people are living together -- married or not -- they tend to share behaviors and activity patterns. For instance, they may cook bigger meals together or eat out more often than when they were single, and may watch TV together instead of going to the gym or playing a sport.

"If this is a time of shifting behaviors and of influencing each other, then maybe it's a good time to intervene with these young couples and get them to have a more positive effect on each other," Gordon-Larsen says. "Maybe they can exercise together or cook healthy meals together. Couples can use that phenomenon (of shared behaviors) to their advantage if they're aware of what's going on."