Sex at Dawn: The Prehistoric Origins of Modern Sexuality is a book dealing with the evolution of monogamy in humans and human mating systems. First published in 2010, it was co-authored by Christopher Ryan and Cacilda Jethá. In opposition to what the authors see as the 'standard narrative' of human sexual evolution, they contend having multiple sexual partners was common and accepted in the environment of evolutionary adaptedness. Mobile self-contained groups of hunter gatherers are posited as the human norm before agriculture led to high population density. According to the authors, before agriculture, sex was relatively promiscuous, and paternity was not a concern, in a similar way to the mating system of Bonobos. According to the book, sexual interactions strengthened the bond of trust in the groups; far from causing jealousy, social equilibrium and reciprocal obligation was strengthened by playful sexual interactions.