Anthropological field research found that people in stone-age environments use arranged marriages for cooperation. Using this information, this book proposes that arranged marriages explain humanity’s unique behavioural and physical traits.An arranged marriage is an exchange of cooperation for reproduction: 'If you cooperate with us, your son can marry our daughter, and our daughter will marry your son'. This marriage system creates a cooperative system requiring no central government or money and a mating system where there is no choice between sexual partners. All individual sexual selection is outside of marriage. This leads to conclusions: love is for extra-marital sexual relationships, married partners did not evolve to be cooperative, human intelligence is based on manipulating extremely complex marriage systems, and people use eye contact to initiate sexual interest because unmarried and unrelated men and women could not freely communicate.This book will redefine humanity as the ape that exchanges cooperation for reproduction.