Have you ever noticed how similar people seem to hang out together? Smokers, broke people, fat people, white people, black people, democrats, republicans, whatever. Vanderbilt and Texas A&M did what I like to call another "could have asked me" study where they figured out the obvious. They could have just asked me and I would have saved them a lot of money because I already knew the answer.
What they found out was that people like to conform to groups, so when a saver hangs out with a bunch of spenders then the saver will slack off and start spending way more money than she normally does. No shit! This "ground breaking" research backs up previous research that shows that fat people make other friends fat, along with smokers and alcoholics.
Now the question is do they have a tendency to assimilate or do they actually bring others down? Both! People of similar nature spend time together, but when a "different" person starts to spend time with them, then they acquire the same bad habits as the group. This is simply human nature. There is safety in numbers and we want to fit in.
What to do about it? If you're trying to lose weight, find health focused friends. If you're looking to save money and grow wealth, find investor/saver friends who put these activities at the forefront of their lives. And regardless of what you're trying to do, never ever hang out with negative people! They will bring you down no matter what you're trying to do.