Not only do you tend to hang out with people like yourself, your friends will influence you toward or away from self-control. Even the people you are forced by circumstances to hang out with (like co-workers) have an influence on your behavior.
That's the finding of researchers who asked participants to watch people either select carrot sticks or cookies to eat before taking tests related to self-control (not involving cookies and carrots). Participants who watched someone eat cookies before the tests did not do as well as those who had watched someone decide to eat carrots.
In another test, participants were told to think of a friend with good self-control. This group performed better on a handgrip test (used to measure self-control) than did the participants assigned to think about a friend with weak self-control. Other tests showed similar results.
Their conclusions: If you surround yourself with people who make wise choices, you are more likely to do the same. You can boost your self-control simply by networking with other people who reinforce positive behavior (or vise versa). And when you show a lack of self-control, you are probably influencing someone else to do the same.
Details of the study are published by the Personality and Social Psychology Bulletin.