Review of E.O. Wilson's new book, Genesis (Clara B. Jones, 2019)

Genesis: the deep origin of societies.

Edward O. Wilson

2019

Liveright Pub. Co. (W.W. Norton & Co.)

153 pp

$15.88

Reviewed by Clara B. Jones, Ph.D. (2019)

"The key to the sociobiology of mammals is milk." EO Wilson (1975)

Social evolution is an important topic of investigation by behavioral ecologists and evolutionary biologists. The two categories of sociality, cooperation and altruism (Hamilton 1964), have arisen infrequently across vertebrate taxa because, in propitious environmental regimes, group-level coordination and control is usually derailed by “cheaters” who fail to comply with group norms. As Wilson pointed out in 1971, groups of cooperators and altruists characterize the most “successful” (i.e., widely distributed) extant terrestrial taxa—social insects and humans. In his new book, Genesis [sic], the entomologist, E.O. Wilson, winner of the Crafoord Prize in 1990 and America's premier social biologist*, assesses the emergence of eusociality, the highest social “grade” (Wilson 1971). 

Perhaps the primary contribution of this brief book is that Wilson classifies humans as eusocial, a system characterized by overlap of generations, cooperative brood-care, and non-reproductive "helpers." If Wilson is correct, humans would be classified, "primitively" eusocial (Wilson, 1971) or Totipotent Eusocial [TE; see "General Mammalian Patterns" blogpost, this blog, #28, 9/19/19], since most human "helpers" (except post-menopausal females or other sterile persons) are expected to be "totipotent"—"helpers" capable of independent reproduction, able to reverse their non-reproductive status [TE]. Members of permanently sterile “castes,” are labeled, “advanced” eusocial (Wilson, 1971), and Wilson's treatments in this book suggest to me that he might be inclined to label permanently non-reproductive human groups as “caste”-like, associations that should be investigated, as well, for the possible presence of "temporal division-of-labor" ("age polyethism": see Wilson, 1971)..

The first five chapters of Genesis include limited explications of some topics (e.g., “multi-level” selection, “phenotypic plasticity”). Wilson clearly explains that the conceptual frameworks of Genesis are Maynard Smith & Szathmáry's (1995) classic treatment of “major transitions of evolution,” as well as, “multi-level” and “group selection,” terms used interchangeably. In Chapter 6, Wilson appears to be primarily interested in proffering a defense for Charles Darwin's explanation for the evolution of sterile castes—an argument based on group selection which Wilson defines as follows: "...within groups, selfish individuals win against altruists, but groups of altruists beat groups of selfish individuals" [attributed to David Sloan Wilson]. Among impediments to "proofs" of "group selection," defenders need to show how "cheaters" are controlled within groups, demonstrations that will require empirical studies.

Here and throughout the book, Wilson fails to incorporate the ecological literature showing, for example, that intragroup competition is generally stronger than intergroup competition or that behavioral ecologists have, since the early 1980s, advanced general criteria for the evolution of cooperative groups (e.g., Emlen 1982) and of eusociality (e.g., Crespi 1994; also see, Choe & Crespi 1997, Crespi et al. 2004 [see, especially, p 66 & pp 73-74], Bourke & Franks 1995, Bourke 2011); West-Eberhard 1975 on p 169, col. 3, par. 2 in Queller & Strassmann 1998 pp 169-170). More specifically, Wilson fails to cite other researchers who have advanced the idea that humans are eusocial (e.g., Foster & Ratnieks 2005, Jones 2011, Crespi 2014).

Nonetheless, combined with related studies (e.g., Emlen 1982, Emlen 1984, Hrdy 2011), there seems to be an expanding literature justifying systematic and quantitative investigation of eusociality in humans, in particular, and in vertebrates, broadly, including, standardization of terminology, experiments, and modeling (e.g., “agent-based” modeling). For an early, published paper that might generate ideas for these future projects, Lotka (1928) is suggested.

The final chapter (7), titled, “The human story,” reviews “transitions” to eusociality across apes, from chimpanzees (Pan troglodytes), as well as, bonobos (P. paniscus) continuing to Australopithecus and the Homo line. Unlike other chapters, this one emphasizes the importance of ecological factors (habitat) for the evolution of social mechanisms among hominids and their ancestors, and Wilson endorses the “social brain hypothesis” as well as the importance of fire for the “rapid evolution” of large brains and the facilitation of group-life, respectively—as well as, their consequent adaptations. Interestingly, in this chapter (p 114), the author compares human eusociality to other social mammals, in particular, African wild dogs, demonstrating that he is prepared to classify “other mammal species,” eusocial, in addition to the social mole rats (see Jones 2014, pp 48-52)****. For another interesting example, see Dwarf Mongoose.

Wilson does not dismiss “kin selection;” but, he holds that “multi-level” or “group” selection is the primary driver of the route to eusociality, behind which kin effects may follow [he may be right; see this lecture, "Ecology Of Societies," by Simon Levin of Princeton:

 https://www.youtube.com/watch?v=rQUsApf3RHs ].

Most social biologists are certain to be surprised to read Wilson's claims that "Hamilton's Rule" suffers "fatal weaknesses" and is no longer "useful (see Bourke 2011, Bourke & Franks 1995)." Wilson does not support these flippant statements with mainstream literature about which there is wide consensus in favor of Hamilton's Rule, and nowhere in his text does he assess assumptions underlying considerations of differential benefits to recipients of social behavior (cooperation or altruism) or differential costs to "donors," terms subsumed in Hamilton's Rule (see, for example, Bourke 2011, Marshall 2015). Related to this, Wilson all but completely avoids optimality [cost-benefit] thinking, and social biologists will, I think, find his explication of group selection obfuscating when applied to genetics, including the assertion that population geneticists have shown the verity of group selection.

Nonetheless, researchers, including, evolutionary psychologists, human biologists, and anthropologists, will derive many testable hypotheses from Wilson's claims, among the more provocative of them, the statement that division of labor by human professional categories is evidence of eusociality and group selection (that they are "caste-like)." I am led to wonder if some human guilds might be characterized by high r (coefficient of relationship), a possibility that would be easy to test. Likewise, guilds, and other formal human groups (e.g., fraternities and sororities, political groups, and the like), should be investigated for evidence of "temporal division-of-labor" ('age polyethism": see Wilson, 1971).

In service to economy, organization, and clarity, Genesis might have been more wisely presented as a "tight" technical paper rather than a manifesto in book form, though Wilson deserves to be applauded for advancing bold ideas, for insisting that human social behavior be subjected to the same analyses that we apply to non-human animals, and that, ultimately, evolutionary explanations will need to be "gene-centered," a long-standing hallmark of Wilson's approach (e.g., Wilson 1975) and that of the heralded evolutionary biologist, Robert Trivers (see, e.g., Trivers 1985 and the remarkable Trivers & Hare, 1976**; also see citations by Bernie Crespi). I recommend this creative and controversial text to specialists, students, and the general audience. It will raise many questions, stimulate thought, and, hopefully, generate conversations*** and research** about variations in human socio-sexual units, as well as, the origins and evolution, the causes and consequences, of group life across all vertebrates.

References

Bourke AFG (2011) Principles of social evolution. Oxford University Press, Oxford.
----, Franks NR (1995) Social evolution in ants. Princeton [NJ] University Press.

Choe JC, Crespi BJ (1997) The evolution of social behavior in insects and arachnids. Cambridge University Press, London.

Crespi BJ (1994) Three conditions for the evolution of eusociality: are they sufficient? Insectes Sociaux 41(4): 395-400.

Crespi BJ (2014) The insectan apes. Human Nature 25(1): 6-27.
Crespi BJ, Morris DC, Mound LA (2004) Evolution of ecological and behavioural diversity: Australian Acacia thrips as model organisms. Australian Biological Resources Study, Canberra.

https://www.environment.gov.au/science/abrs/publications/thrips

Emlen ST (1982) The evolution of helping I: an ecological constraints model. American Naturalist 119: 29-39.

Emlen ST (1984) Cooperative breeding in birds and mammals. Pp 305-339 in Behavioral ecology an evolutionary approach, 2^nd ed. (JR Krebs, NB Davies, eds.). Sinauer, Sunderland, MA.
Emlen ST, Oring L (1977) Ecology, sexual selection, and the evolution of mating systems. Science 197: 215-223.

Foster KR, Ratnieks FLW (2005) A new eusocial vertebrate? Trends in Ecology and Evolution 20(7): 363-364.
Frank SA (1995) Mutual policing and repression of competition in the evolution of cooperative groups. Nature 377: 520-522.
----(2003) Repression of competition and the evolution of competition. Evolution 57(4): 693-705.

Hamilton WD (1964) The genetical evolution of social behavior. Journal of Theoretical Biology 7: 1-52.
Holldobler B, Wilson EO (1990) The ants. Belknap (HUP). Cambridge.

Hrdy SB (2011) Mothers and others. Belknap-Harvard.

Jones CB (2011). Are humans cooperative breeders? A call for research. Archives of Sexual Behavior 40(3): 479-481.
----(2014) The evolution of mammalian sociality in an ecological perspective. Springer, NY.

Lotka AJ (1928) Sterility in American marriages. PNAS 14(1): 99-108.

Marshall JAR (2015) Social evolution and inclusive fitness theory: an introduction. Princeton University Press, Princeton, NJ.

Maynard Smith J, Szathmáry E (1995) The major transitions of evolution. W.H. Freeman Spektrum, New York.

Queller DC, Strassmann JE (1998) Kin selection and social insects. BioScience 48(3): 165-175.
Trivers RL (1985) Social evolution. Benjamin-Cummings Pub. Co., San Francisco.
Trivers RL (2015) Wild Life. Biosocial Research. New Brunswick, NJ. [& e-book, amazon.com] 

http://vertebratesocialbehavior.blogspot.com/2019/04/review-of-robert-l-trivers-memoir-wild.html

----, Hare H (1976) Haplodiploidy and the evolution of the social insects. Science 191(4224): 249-263.

West-Eberhard MJ (1975) The evolution of social behavior by kin selection. Quarterly Review of Biology 50: 1-33.
Wilson EO (1971) The insect societies. Belknap/Harvard, Cambridge, MA.

----(1975) Sociobiology. Belknap/Harvard, Cambridge, MA.
Wittenberger JF (1980) Group size and polygamy in social mammals. Am. Nat. 115: 197-222.
Yamamura N, Higashi M (1992) An evolutionary theory of conflict resolution between relatives: altruism, manipulation, compromise. Evolution 46: 1236-1239.

*First, no American social biologist can compete with Wilson's expertise as a student of a single social group, ants, in Wilson's case. Second, IMO, The Insect Societies (1971) is the greatest technical book ever written to date (and The Ants [Holldobler & Wilson, 1990] the greatest popular book ever written to date) in Animal Behavior and Ethology.

Third, unlike E.O. Wilson, Robert L. Trivers, also a renowned social biologist, winner of the Crafoord Prize in 2007, is not a synthesizer, though Trivers' most heralded papers have broad import. Trivers has not communicated much interest in a search for general patterns--within, between, and across taxa. Also, again, in contrast to E.O. Wilson, Trivers' canon pays scant attention to Genotype<----->Phenotype<----->Environment<-----> causes and effects. One seeks, for the most part, in vain, to locate Ecology--abiotic & biotic Environments (however conceptualized)--in Trivers' writings. Furthermore, to my knowledge,

Trivers has not emphasized the topics--group-formation, group maintenance, and the environmental conditions facilitating the emergence of sociality (which may or may not follow group-formation). Especially pertaining to the latter are the topics, competition and differential access to limiting resources, as well as, limiting resource dispersion (distribution and abundance in time and space). A litany of Behavioral Ecology is that, in some environmental regimes, kin may be ego's worst enemy. Such conditions may occur, for example, where there is intense local competition for limiting resources [above some critical threshold of decreasing reproductive gains].

Trivers may be one of the last remaining extreme genetic thinkers among living social biologists (see my review of his memoir, Wild Life [2015] linked above). He typically asks a question, then, considers what consequences would obtain given alternate, pairwise combinations of related individuals [parents, full sibs: 1/2; first cousins: 1/8, etc.]. Trivers' canon is about mechanisms, not, about causes. His approach has yielded several fundamental papers; however, Trivers' work does not satisfactorily address variations in inter-individual interactions nor evolution in heterogeneous regimes nor phenotypic plasticity nor the principle that behavior is condition-dependent nor the litany of Behavioral Ecology that patterns of  group-living will "map" onto dispersion [distribution and abundance in time & space and abiotic phenomena (e.g., climate)] of limiting resources nor the ideas that females are energy maximizers--males, time minimizers. Trivers does not seem to be sensitive to Hamilton's "b" & "c" whereby an actor & a recipient, whatever their "r" [coefficient of relationship], will respond relative to "b" [benefits to recipient] and "c" [costs to actor ("donor"--of reproductive units)], rather than, strictly, "r" [Hamilton's Rule: rb - c >0].

Trivers' literal logic based, apparently, on "r" alone, may reveal one unfortunate consequence of the term, "kin selection" that leads many to assume that it is always in ego's favor to exhibit social behavior towards kin. The latter assumption may be an assumption behind Trivers' (very successful and justifiably heralded) publications. Furthermore, as I [and several others before me (thanks to James Marshall and Andrew Bourke for making me aware of this literature)] have suggested, it may be useful to consider the role of competition influencing behavior between actor [donor] and recipient (see Yamamura & Higashi, 1992)  and to question whether Hamilton's Rule adequately incorporates the consequences for actor and recipient and for the expression or non-expression of cooperation or altruism (i.e., "social behavior") of interindividual competition for limiting resources (e.g., food, mates, space, etc.).

Thus, sometimes, kin may be ego's "worst enemy" [it may not be beneficial for ego to assist the reproduction of kin; it may be in the interest of ego to assist the reproduction--depending upon environmental regime] where predation is non-random by genotype [where cooperation or altruism toward a relative would increase ego's chances of becoming prey]. But, complicating the matter, in certain conditions, death, however, defined [e.g., self-induced, other-induced], can benefit kin. Clearly, systematic empirical and theoretical studies, in addition to modeling, are needed.

In the final analysis, however, the impressive success of Trivers' verbal models based on "r" may demonstrate the power of Hamilton's Rule to predict a very broad array of the social acts (cooperation, altruism) observed in Nature, including, Human Nature. However, we should not only ask, "What is "r"?, but, also (or rather?), "'r' relative to what?" According to Hamilton's Rule, the effects of "r" are expected to be constrained by the factors comprising "b" & "c". For example, when and under what conditions are kin, enemies?

**See comments on this paper in Holldobler & Wilson (1990, p 184)

***For example, cooperation and altruism might occur by way of "mutual policing," coercion, force, or persuasion, in addition to, "self-restraint" (see, especially, Steve Frank's work, in particular. 1995 and 2003).

****Addendum 4/26/2019: Regarding the ideas that humans are "cooperative brreeders" (e.g., Hrdy 2011) and that "cooperatively-breeding" mammals should be classified, "eusocial" (Jones 2014), I am going to go out on a limb to suggest that, based upon my reading to date on mammal, including, human, group-living patterns, mammalian "cooperative breeding" evolved from groups of communal females, and their offspring, that may or may not, then, evolve to share "economic" tasks, including care of young [see epigraph]...since females, ceteris paribus, are expected to be "energy-maximizers," thermal efficiency, broadly defined, will be paramount as a selective factor and in determining optimal "inclusive fitness maximizing" (see Jones 2014, Chapter 3). Anthropologists specializing in human "cooperative breeding" hold that the system evolved via monogamy, as it is thought to have done in insects and birds. However, mammalian sociosexual systems generally exhibit "sexual segregation" ("solitary" dispersion between the sexes) and, in mammals, monogamy is derived. Related, Mammal sociosexual systems and group structure are thought to result from the tendency of females to select rich patches of [limiting] food and that of males to select the largest possible number, sometimes, an aggregation, of females (Wittenberger 1980, Emlen & Oring 1977).

In summary, it seems to me that the more likely "route" to eusociaity in humans would have been via communally nesting mammalian ancestors, particularly, since monogamy is "derived," mammals are very likely to exhibit a "sexually-segregated" ["solitary"] socio-sexual organization, monogamy is relatively rare, and, in humans, at least, monogamy is often imposed by some authority. Routes to eusociality that might be applied to humans have, also, been proposed by West-Eberhard (1975) and by Queller & Strassmann (1998: "fortress defenders" and "life insurers") [see Queller & Strassmann (1998) for a brief summary of these ideas on pp 169-170]. These issues need to be unpacked.

Clara B. Jones is a retired behavioral ecologist living in Silver Spring, MD (USA). 

Are Humans Cooperative Breeders? [2]

Are humans cooperative breeders? [2]

"Given the high diversity of ecological and social environments within which humans are currently found, specific human groups, populations, or cultures, rather than the human species as a whole, should be considered as units for comparison, analysis, and discussion." (Crespi 2014; also see Jones 2011)

Based upon Sarah Blaffer Hrdy's influence, it has become fashionable in the literature of Anthropology, to term [and, by implication, to classify] humans as "cooperative breeders". The purpose of this blogpost is to highlight Crespi's (2014) recent treatment of this topic and to provide, hopefully, constructive criticism of the topic's most recent discussion (Kramer & Russell 2015). In particular, I will make explicit certain untested assumptions underlying the classification as well as an apparent misunderstanding and/or mis-application of Hamilton's rule. This diagnosis is not intended to be exhaustive; however, it is my opinion that the concerns raised here provide a framework for necessary research to be conducted before the variety and range of conformations of human reproductive units can be described, including, traits associated with different conformations within and between populations.

In particular, I wish to stress that allomothering/alloparenting may be imposed by exploitation in the form of, for instance, coercion or social parasitism by reproductives [mothers] upon Donors [Actors, "helpers", "allomothers"] and that, rather than benefiting reproductive interests of Recipients [mothers] AND Donors ["helpers", "alloparents", "allomothers"], as per Cooperation, apparent "cooperation" may be deleterious to Donors/"allomothers" who may exhibit Altruism and/or apparent "cooperation" may be induced/imposed by Selfish behaviors on the part of Recipients [mothers], e.g., via coercion, including, in some cases, manipulation via financial reward.

Kramer & Russell (2015) recently addressed monogamy as a possible precursor to "cooperative breeding" in humans, explicating, clearly, and without qualification, the current, accepted argument[s] in Anthropology for classifying, without qualification, the human species as "cooperative breeders" [see epigram to this blogpost]. The following issues of concern pertain to the aforementioned paper and to Hrdy's earlier claims (Hrdy 1999, 2005, 2009 cited in Crespi 2014 and Kramer & Russell 2015 [subsequently, K&R]).

1. K&R define "social [sic] breeding systems" as "cooperative breeders" exhibiting "two key characteristics" [pg 74]: (1) "those individuals who provide care to the offspring of others [and are] currently non-breeders" and (2) "helping behaviors may be directed to another's offspring or toward mothers or other caretakers, which facilitates increased investment in offspring now or in the future." There is little consensus among researchers about what characterizes "cooperative breeding"; thus, the criteria advanced by K&R require comparative analysis and may be considered tentative hypotheses. An obvious problem confronting the aforementioned two criteria is that many species not classified as "cooperative breeders" meet the two conditions [e.g., grooming or "allomothering" in many primate and social carnivore taxa such as macaques, baboons, wild dogs, and hyenas or numerous "communal" rodents].

2. Throughout K&R, and much [all?] of the literature on [apparent, ostensible] "cooperation" in the Anthropology literature, apparent, ostensible "cooperation" is assumed to be a positive interaction [facilitation]. This is a non-trivial, untested and implicit assumption. "Helping" cum "cooperation" requires measurement or estimation of differential [reproductive] costs and benefits associated with "allomothering" behaviors [traits] before we can label these interactions "cooperative". Further, Cooperation may be imposed upon Donors ["helpers", "allomothers"] by Recipients [mothers] by a wide range of exploitation and manipulation [negative interactions], such as coercion, force, persuasion, social parasitism, shaping, and/or financial reward.

3. Related to #2 above, K&R, and other Anthropologists, appear to assume that benefiting kin is generally, if not always, advantageous to Donor [mother]. This assumption suggests a fundamental misunderstanding of Hamilton's Rule which does not state that benefits to [potential] Donor will always be + since costs of doing so may be prohibitively high. This fundamental error highlights the importance of measuring or estimating differential costs and benefits to Donor ["allomother"] and Recipient [mother] alike.

On the other hand, induced or imposed [potential] costs to [potential] Donor may make it beneficial for Donor to "help" where it is not possible for [potential] Donor to escape costs, such as, where response[s] to [potential] Recipient [mother or other self-interested group member"/imposer" of costs] are not under [potential] Donor's ["allomother's"] control. [Reproductive] costs from NOT "helping"/"allomothering" may be prohibitively high for [potential] Donor if s/he can not escape control by [potential] Recipient [mother].

4. After K&R, "Humans live in multi-male, multi-female groups that include multiple breeding females who often reside within socially recognized, long-term monandrous unions." [p 75]. These conditions may characterize some other mammalian groups, populations, or taxa, a topic that remains unstudied systematically.

5. Two additional criteria that have not, to my knowledge, been addressed by Anthropologists relative to their classification of humans as "cooperative breeders" is the extent to which the moniker requires high "skew" in groups and totipotent "helpers" [non-reproductive "allomothers" who may revert, in a condition-dependent, situation-dependent manner, to reproductives]. Should the latter trait be agreed upon as a diagnostic trait for classification of a group or population to be considered "cooperatively-breeding", menopausal females [often assumed to be grandmothers] and pre-reproductive offspring would not be considered diagnostic for said classification. See, for example, literature on "cooperatively-breeding" callitrichids for diagnostic use of high "skew" and totipotency, in addition, to employment of these terms in literature on Social Insects (see, also, Crespi 2014).

6. I am not aware of any quantitative [especially, modelling or theoretical] treatments of the above topics nor of experiments [lab, naturalistic, agent-based modeling] on the aforementioned matters in the Anthrropology literature.

7. I intend for the previous comments to convince the reader of the need for additional research within and between human groups in order to systematically evaluate the range and varieties of reproductive units in humans, a species with noteworthy facultative behavioral responses (see Jones 2011 and "Are humans cooperative breeders? [1], this Blog, as well as epigram above from Crespi 2014; also see Crespi 2009).

8. Following the epigram to this blogpost and related literature cited, it is clear that it is ill-advised to label humans as a species characterized by a single "breeding" system. Furthermore, without further evidence, it is not clear that "cooperative breeding" is the norm in most human populations over time and space. Related to this, there is no consensus among mainstream researchers concerning diagnostic criteria for a cooperatively-breeding group or population.

9. In accord with the call for additional research, there is a need to evaluate the literature on deleterious effects of "allo"-carers upon dependent human young. For example, male caretakers may injure or kill young at alarming rates in human groups and populations. Presumed benefits of grandmothers may be exaggerated and their deleterious effects may be minimized.

10. In 1999 (Jones & McJetters 1999), I wrote the following as a Note [p 123, #s 2 & 3] in a paper about women on death row. The statement suggests ways to quantitatively address the Socioecology of Co[-]operative Breeding in Humans. The significance of the Notes is unrelated to the topic of the paper [women on death row]. One significance of the Notes is to highlight Crespi's (2014; Jones 2011) statement  above [epigram to this blogpost]...human mating systems are expected to vary, and the causes and consequences of such variation may differ as a function of, say, SES [class], race, etc. Another point of interest [Note 3] is expected to be the dynamics of interactions between mothers and potential or actual mates.

Note 2: "We believe that Emlen's (1995) ecological model of the family will facilitate the analysis of behavioral differences between Black and White women, including differences in violent behavior. Emlen's ecological model predicts cooperative family organization (e.g., extended families) in regimes where independent breeding is uncertain, as is the case for many Black females. Where independent breeding is more reliable, as it may be for most White women, individualistic strategies are predicted. Ray & Smith (1991, p 150) also note the importance of ecological models for analyzing the consequences of differential access to resources in time and space for homicidal behavior. Ecological models are potentially of value to the student of violence because they are sensitive to interindividual competition for limiting resources."

Note 3: "A complicating factor, however, is that the profile of Black female homicide offenders is expected to be tied to that of Black males, since ecological theory predicts that female behavior will be a function of their resource base, including potential investment by males in herself, her childre, and her kin. Since the social and economic status of Black males is expected to be lower on average than that of White males, the causes and consequences of criminal behavior by Black and White women may continue to differ significantly."

Crespi (2009)

http://www.sfu.ca/biology/faculty/crespi/pdfs/122-Crespi2009SkewTheory.pdf

Crespi (2014)

http://www.sfu.ca/biology/faculty/crespi/pdfs/168-Crespi2013HumanNature.pdf

Emlen (1978)

Emlen ST (1978) The evolution of cooperative breeding in birds. In: Krebs JR, Davies NB, Behavioural ecology: an evolutionary approach. Blackwell, UK.

Hrdy (1976)

https://books.google.com/books?hl=en&lr=&id=x_hfmoOaHpoC&oi=fnd&pg=PA101&dq=sb+hrdy++1976&ots=IrAlXVg1ae&sig=OZrcPETIUNcFezBBbxBVssldVR4#v=onepage&q&f=false

Jones (1986)

http://onlinelibrary.wiley.com/doi/10.1002/1098-2337(1986)12:3%3C167::AID-AB2480120303%3E3.0.CO;2-X/abstract

Jones (2011)

http://link.springer.com/article/10.1007/s10508-011-9741-5#page-1

Jones CB, McJetters Y. (1999) Gender, race, and homicide: a preliminary analysis. The Western J of Black Studies 23:2, 119-124.

Kramer & Russell (2015)

http://onlinelibrary.wiley.com/doi/10.1002/evan.21445/abstract

Solomon & French (1997)

Solomon NG, French JA (1997) Cooperative breeding in mammals. CUP, UK.

Are Humans Cooperative Breeders? [1...my 6 min video]

https://www.youtube.com/watch?v=Cvz3D3sKlJ8&feature=youtu.be