Hmm... I'd be inclined to disagree. "More likely" doesn't mean "Guaranteed positive gain." And, I think it would be faulty to assume that physical gain is the only form of gain a person might get... Psyche plays a big part as well. =)
Someone who donates to Toys for Tots will feel good that the toys they donate will go towards kids who wouldn't have something to open for Christmas. When someone does a good deed for someone, they remember that and are more likely to return the deed in the future, or they may feel inspired to do something nice for someone else. Positive exertion in the world will more likely influence the world (or yourself) in a positive way.
When someone smokes meth, they are more likely to become addicted to it, and they are more likely to start down a self destructive path... And when people break their conscience, and do something they feel is wrong, they will damage their perceptions of themselves, incur a debt of guilt, or will simply tune out that inner compass... And when you do that, eventually it will stop guiding you altogether, and that will hinder your ability to interact with others in a cohesive way... which makes it more likely for someone to form a grudge against you. Negative exertion in the world will more likely influence the world (or yourself) in a negative way.
Is it guaranteed? No... But the decisions we make DO have consequences. =)