There has been a lot of research recently into the effects of a hormone called oxytocin which is believed to be linked to positive emotions and social interaction. A study of healthy male volunteers gave them either oxytocin or a placebo and presented them with a series of happy, angry and neutral faces on a computer screen. The participants returned the following day when they were presented with another set of faces and asked to pick out the ones they had seen the day before. Those people who had been given oxytocin were more likely to remember the happy faces rather than the angry and neutral ones.
You can find out more about this research at