It is debated whether this redirection of human nurturing behaviour towards non-human animals, in the form of pet-keeping, was maladaptive, due to being biologically costly, or whether it was positively selected for. Two studies suggest that the human ability to domesticate and keep pets came from the same fundamental evolutionary trait and that this trait provided a material benefit in the form of domestication that was sufficiently adaptive to be positively selected for. A 2011 study suggests that the practical functions that some pets provide, such as assisting hunting or removing pests, could have resulted in enough evolutionary advantage to allow for the persistence of this behaviour in humans and outweigh the economic burden held by pets kept as playthings for immediate emotional rewards. Two other studies suggest that the behaviour constitutes an error, side effect or misapplication of the evolved mechanisms responsible for human empathy and theory of mind to cover non-human animals which has not sufficiently impacted its evolutionary advantage in the long run.