Blackmailing: the keystone in the human mating system

Abstract

Background: The human mating system is characterized by bi-parental care and faithful monogamy is highly valued in most cultures. Marriage has evolved as a social institution and punishment for extra pair mating (EPM) or adultery is common. However, similar to other species with bi-parental care, both males and females frequently indulge in EPM in secrecy since it confers certain gender specific genetic benefits. Stability of faithful monogamy is therefore a conundrum. We model human mating system using game theory framework to study the effects of factors that can stabilize or destabilize faithful committed monogamy.

Results: Although mate guarding can partly protect the genetic interests, we show that it does not ensure monogamy. Social policing enabled by gossiping is another line of defense against adultery unique to humans. However, social policing has a small but positive cost to an individual and therefore is prone to free riding. We suggest that since exposure of adultery can invite severe punishment, the policing individuals can blackmail opportunistically whenever the circumstances permit. If the maximum probabilistic benefit of blackmailing is greater than the cost of policing, policing becomes a non-altruistic act and stabilizes in the society. We show that this dynamics leads to the coexistence of different strategies in oscillations, with obligate monogamy maintained at a high level. Deletion of blackmailing benefit from the model leads to the complete disappearance of obligate monogamy.

Conclusions: Obligate monogamy can be maintained in the population in spite of the advantages of EPM. Blackmailing, which makes policing a non-altruistic act, is crucial for the maintenance of faithful monogamy. Although biparental care, EPM, mate guarding and punishment are shared by many species, gossiping and blackmailing make the human mating system unique.

Figure 1

Invasion dynamics of 8 pure strategies. Black arrows indicate that invasion is possible through basic assumptions of the model. Grey arrows indicate that invasion is possible only under certain conditions. Arrows point at the invading strategy. The circled strategy is an unstable equilibrium. The hollow arrow indicates that the payoff of a lone invader is the same as the population, however even with slight drift invasion is possible.

Figure 2

Vector field plots of differential equations (2), (3) and (4). (A) in the absence of punishment and blackmailing (y = 0, a = 0), (B) in the presence of punishment but no blackmailing (y > 0, a = 0) and (C) in the presence of punishment and blackmailing (y > 0, a > 0). Arrows indicate the direction of change in strategy. Grey circle indicates unstable equilibrium and black circle indicates stable equilibrium. (A) and (B) end up at the stable equilibrium (0,1,0) whereas (C) leads to cyclic oscillations in strategies with no ESS. Other parameter values are l = z = 0.2, α = β = 0.2, c_P = 0.01, c_G = 0.02, y = 0.1, s = 0.4 and a = 0.08.

Figure 3

Blackmailing leads to cyclic oscillations in the mixed strategies in the presence (A and B) and absence (C and D) of mate guarding. (A) In the presence of mate guarding, all three traits, i.e. monogamy, mate guarding and policing show stable oscillations which form limit cycle (B) in p-q-r space. (C) In the absence of mate guarding monogamy and policing still oscillate in stable oscillations, which form a limit cycle (D) in p-r space. All parameter values are as per Figure 2 except in (C) and (D) y = 0.2, s = 0. Arrows indicate direction of change.

Figure 4

Variations of the model to incorporate constraints on blackmailing opportunity, where (a) blackmailing opportunity decreases with increase in the mate guarding, and (b) blackmailing opportunity decreases with increase in policing individuals. In both the cases, the results are qualitatively similar in that blackmailing leads to cyclic oscillations in strategies with no ESS similar to Figure 2C. Arrows indicate the direction of change in strategy. All parameter values as per Figure 2.