title: An evolutionary Ockham's razor to reciprocity.
authors.affiliation: Department of Cognitive Science, Educational and Cultural Studies, University of Messina Messina, Italy ; Department of Social Psychology, University of Amsterdam Amsterdam, Netherlands.
authors.forename: Irene
authors.initials: I
authors.lastname: Berra
keywords: emotional bookkeeping; endorphins; oxytocin; proximate vs. ultimate causes; reciprocity; reward; social bonding