A team is only as fast as its slowest member—and a couple is only as thrifty or healthy as its less thrifty, less healthy half. A study in the Journal of Consumer Research found that when both partners have a high degree of restraint, the couple has a relatively easy time saving money, buying healthy foods and keeping on-task. When both people have low-self control, as you might expect, it’s the opposite. And when only one partner has self-control, he or she tends to defer to the other’s impulses. In fact, the researchers found that these pairs behave just like couples with no self-control between them.