I came across this video on Facebook last week (first watch it, to avoid spoilers further in the text):
It is based on the famous Prisoner's Dilemma, which the host of the show explains nicely. Richard Dawkins also devoted some pages on this in his breakthrough-book The Selfish Gene. It turns out there are always cheaters (like this women) who can take advantage of cooperators (like the poor guy). And it is not just restricted to humans. I recently wrote two articles for a Dutch popular science website, one on bacteria and one on fruitflies. In both cases there was cheating involved.
So, the bottom line is...don't trust anyone!