Her hypothesis is that teens behave differently depending on how hostile and unforgiving their local environment feels to them, an idea that social scientists called "life history theory". In tougher times in history, teens were forced to take a "fast life strategy", growing up faster, reproducing earlier and focusing on basic needs. Now life in the West is generally more forgiving, and families are wealthier – at least on average – so it"s possible for teens to take a "slow life strategy", delaying the transition to more adult behaviours.