Ryan M. Ellsworth

Evolutionary History of Hunter-Gatherer Marriage Practices

Background The universality of marriage in human societies around the world suggests a deep evolutionary history of institutionalized pair-bonding that stems back at least to early modern humans. However, marriage practices vary considerably from culture to culture, ranging from strict prescriptions and arranged marriages in some societies to mostly unregulated courtship in others, presence to absence of brideservice and brideprice, and polyandrous to polygynous unions. The ancestral state of early human marriage is not well known given the lack of conclusive archaeological evidence. Methodology Comparative phylogenetic analyses using data from contemporary hunter-gatherers around the world may allow for the reconstruction of ancestral human cultural traits. We attempt to reconstruct ancestral marriage practices using hunter-gatherer phylogenies based on mitochondrial DNA sequences. Results Arranged marriages are inferred to go back at least to first modern human migrations out of Africa. Reconstructions are equivocal on whether or not earlier human marriages were arranged because several African hunter-gatherers have courtship marriages. Phylogenetic reconstructions suggest that marriages in early ancestral human societies probably had low levels of polygyny (low reproductive skew) and reciprocal exchanges between the families of marital partners (i.e., brideservice or brideprice). Discussion Phylogenetic results suggest a deep history of regulated exchange of mates and resources among lineages that enhanced the complexity of human meta-group social structure with coalitions and alliances spanning across multiple residential communities.

Human Female Orgasm as Evolved Signal: A Test of Two Hypotheses

We present the results of a study designed to empirically test predictions derived from two hypotheses regarding human female orgasm behavior as an evolved communicative trait or signal. One hypothesis tested was the female fidelity hypothesis, which posits that human female orgasm signals a woman’s sexual satisfaction and therefore her likelihood of future fidelity to a partner. The other was sire choice hypothesis, which posits that women’s orgasm behavior signals increased chances of fertilization. To test the two hypotheses of human female orgasm, we administered a questionnaire to 138 females and 121 males who reported that they were currently in a romantic relationship. Key predictions of the female fidelity hypothesis were not supported. In particular, orgasm was not associated with female sexual fidelity nor was orgasm associated with male perceptions of partner sexual fidelity. However, faked orgasm was associated with female sexual infidelity and lower male relationship satisfaction. Overall, results were in greater support of the sire choice signaling hypothesis than the female fidelity hypothesis. Results also suggest that male satisfaction with, investment in, and sexual fidelity to a mate are benefits that favored the selection of orgasmic signaling in ancestral females.

Relatedness, Co-residence, and Shared Fatherhood among Ache Foragers of Paraguay

Hypotheses on the benefits of the practice of partible paternity are tested using demographic data for Ache foragers of Paraguay. Partible paternity refers to the institution of multiple males considered to contribute to the conception of a single offspring. Analyses focus on patterns of primary and secondary co-fatherhood among men, genealogical relationships between co-fathers, and relation between band co-residence and co-fatherhood. Results indicate that men who had more secondary fatherhood also had more primary fatherhood; co-fathers are more closely related, on average, than men who are not co-fathers; and co-fathers were also more likely to reside together than men who were not co-fathers, even after controlling for relatedness. Results are most consistent with women choosing co-fathers of offspring in ways that maximize likelihood and amount of investment (multiple investors hypothesis) and men competing for more mates with at least partially affiliative outcomes (mate competition and male alliance hypotheses).

Talk and Tradition: Why the Least Interesting Components of Religion May Be the Most Evolutionarily Important

Many evolutionary explanations of religion tend to focus on the strange and exotic aspects of religion assumed to be the result of weird beliefs in things that do not exist. In contrast, we propose that religious behavior is distinguished from non-religious behavior by one of the most familiar and mundane of human behaviors: talk. Specifically, we suggest that it is the making, and communicating acceptance, of supernatural claims that distinguishes religious behavior. Given that talk is a form of communication aimed at influencing the behavior of others, we propose that proximate explanations of religion focus on the identifiable social effects of religious behavior. Given that the talk identified as religious was almost certainly transmitted from ancestors to descendants during recent human evolution, we suggest that ultimate explanations of religion focus on the consequences of traditional religious behavior transmitted through a multitude of generations. Our hypothesis is that religious traditions were perpetuated because they increased cooperation among co-descendants by communicating the willingness to accept the influence of each other and their common ancestors.

Book Review: The Myth of Promiscuity

The book Sex at Dawn was published in 2010 and quickly became a best-seller, receiving kudos from well-known personages such as sex advice columnist Dan Savage, and primatologist Frans de Waal (Savage calling it the ―most important book on human sexuality‖ since Kinsey‘s 1948 Sexual Behavior of the Human Male 1 ; de Waal dubbing it an ―exciting book‖ that raises issues that will ―need debating over and over‖ 2 ). Sex at Dawn appears to have struck a chord with a certain starryeyed segment of the reading public, as well as some academics who should know better. For those unfamiliar with Sex at Dawn (Ryan and Jethá, 2010), the main thrust of the book is its claim that, contrary to conventional scientific wisdom—called the ―standard narrative of evolutionary psychology‖—pair-bonding, sexual jealousy, a male concern with paternity certainty, and host of other related traits are not ―natural‖ components of evolved human sexuality. Rather, they are the product of the social arrangements attending the emergence of agriculture beginning only about 10,000 years ago. Our true nature, the authors of Sex at Dawn argue, is one closer to that of what they think bonobo sexuality is, i.e., fluid, promiscuous sexual relations between all individuals, with little sexual conflict to speak of. Ryan and Jethá argue that the evidence points to the conclusion that promiscuous sexuality characterized our ancestral hunter-gatherer past, and that those evolutionary scientists who formulated and uphold the ―standard narrative‖ are mistakenly projecting modern, post-agricultural mores onto our ancestors as well as contemporary small-scale societies. While the book continues in its lay popularity, it has not achieved a position of respect, or even much attention, from researchers who would likely be associated with the so-called standard narrative. A call to arms suggested in a review of Sex at Dawn has gone

Comparative Study of Reproductive Skew and Pair-Bond Stability Using Genealogies from 80 Small-Scale Human Societies

Genealogies contain information on the prevalence of different sibling types that result from past reproductive behavior. Full sibling sets stem from stable monogamy, paternal half siblings primarily indicate male reproductive skew, and maternal half siblings reflect unstable pair bonds.

Co-father relationships among the Suruí (Paiter) of Brazil

Partible paternity refers to the conception belief that children can have multiple fathers (“co-fathers”) and is common to indigenous cultures of lowland South America. The nature of social relationships observed between co-fathers reveals information about the reproductive strategies underlying partible paternity. Here we analyze clan, genealogical, and social relationships between co-fathers for the Suruí, an indigenous horticultural population in Brazil. We show that co-fathers roughly assort into two separate categories. In the affiliative category, co-father relationships are amicable when they are between close kin, namely brothers and father-son. In the competitive category, relationships are more likely of avoidance or open hostility when between more distant kin such as cousins or unrelated men of different clans. Results therefore imply multiple types of relationships, including both cooperative and competitive contexts, under the rubric of partible paternity. These complexities of partible paternity institutions add to our knowledge of the full range of cross-cultural variation in human mating/marriage arrangements and speak to the debate on whether or not humans should be classified as cooperative breeders.

Reply to Weight and Keefe

While the careful attention paid to our article “Relatedness, Co-residence, and Shared Fatherhood among Ache Foragers of Paraguay” (Ellsworth et al. 2014) by Weight and Keefe (2016) is appreciated, their commentary is somewhat puzzling. The authors are not in disagreement with the findings on patterns of co-residence and relatedness among Ache co-fathers, nor are they in disagreement with any interpretations thereof explicitly made in the article. The sole concern of Weight and Keefe’s commentary appears to be an out-of-hand dismissal of any consideration of fitness benefits to women in formulating tentative explanations of the origin and maintenance of co-father investment. This is made clear by their statement that “fitness consequences to mothers from co-paternal care should not be used to inform the evolution of co-paternal care.” Rather, they choose to focus attention only on fitness benefits to men of providing such care. Likening co-father care of and affiliation with a woman and her offspring to male-female “friendships” as observed in some nonhuman primates, such as baboons, Weight and Keefe argue that investment in mothers and their offspring by secondary fathers is best conceptualized as mating effort, and they suggest that “Ache co-fathers develop relationships with mothers to increase their mating opportunities.” But secondary fathers are secondary fathers precisely because they had prior sexual relations with a child’s mother. In contrast to the young male baboons that the authors describe, who affiliate with a female and her infant to increase their opportunity to

Book Review: Dr. Strangeape, or How to Learn to Stop Worrying and Love the Bonobo

(Ryan and Jetha, 2010) comes another effort to place bonobos on center stage in the study of human nature. Sex therapist and talk show host Susan Block has written a self-help book that offers to improve our lives through the “alternative great ape paradigm” (p. 55) of peace, love, and sexual liberation represented by the bonobo. In