As well as radiation and loss of bone density and muscle atrophy on long duration space missions, there is the boredom and psychological distress of social isolation.
as this article puts it:
http://www.economist.com/science/dis...ory_id=9401568
"Some psychologists propose sending an all-female crew to Mars. Even if women become irritable, they are less likely to commit suicide or murder each other than men are. Others think a mixed team would support each other better. But that, as the European experiment may demonstrate, raises the possibility of the first human Martians. Perhaps it would be better to stick to more psychologically robust and less libidinous space explorers: robots."