Infidelity Many human societies are based around notionally monogamous relationships However, it is relatively common for both men and women to engage in sexual activity with additional partners

Infidelity Prevalence of sexual infidelity difficult to measure for obvious reasons. Some estimates from the US: Sexual infidelity occurs in 20-25% of marriages (Wiederman, 1997) Sexual infidelity occurs in 20-25% of marriages (Wiederman, 1997) 65-75% of university students have had some degree of ‘extradyadic involvement’ whilst in a ‘serious’ relationship (Shackelford et al, 2000) 65-75% of university students have had some degree of ‘extradyadic involvement’ whilst in a ‘serious’ relationship (Shackelford et al, 2000)

Infidelity Infidelity also occurs in various animal species, including those that apparently form monogamous pair bonds It occurs in both males and females It occurs in both males and females E.G. in some supposedly monogamous bird species 10-40% of chicks were fathered by a male other than the female’s pair-bonded mate E.G. in some supposedly monogamous bird species 10-40% of chicks were fathered by a male other than the female’s pair-bonded mate

Infidelity & Evolution Infidelity might have evolutionary advantages & be an adaptive strategy If so, male & female infidelity are likely to have different motives & possible consequences

Male Infidelity Possibly explainable in terms of ‘quantity’ based indiscriminate mating strategy More partners = more offspring Men more likely to report having affairs that were ‘just about sex’ (Glass & Wright, 1985) Men more likely to report having affairs that were ‘just about sex’ (Glass & Wright, 1985) Male infidelity not necessarily linked to dissatisfaction with current relationship (Hall & Fincham, 2005) Male infidelity not necessarily linked to dissatisfaction with current relationship (Hall & Fincham, 2005)

Female Infidelity Possibly explainable in terms of ‘quality’ based strategy for optimising survival chances of offspring Best physical specimens may not be best resource providers (‘best of both worlds’) Best physical specimens may not be best resource providers (‘best of both worlds’) Infidelity more strongly linked to dissatisfaction with currently relationship than in male infidelity (Glass & Wright, 1985) Infidelity more strongly linked to dissatisfaction with currently relationship than in male infidelity (Glass & Wright, 1985)

Infidelity That infidelity serves different purposes in M & F is supported by other evidence e.g. M less likely to forgive, more likely to break up with sexually, rather than emotionally unfaithful partner (Shackelford et al, 2002) M less likely to forgive, more likely to break up with sexually, rather than emotionally unfaithful partner (Shackelford et al, 2002) F seem more sensitive than M to emotional infidelity (Hall & Fincham, 2004) F seem more sensitive than M to emotional infidelity (Hall & Fincham, 2004)

Infidelity & Parental Investment Infidelity may have an influence on investment in offspring & other parental behaviour It makes poor evolutionary sense to invest in offspring that don’t carry your genes It makes poor evolutionary sense to invest in offspring that don’t carry your genes Females can be certain that the young are theirs, males less so Females can be certain that the young are theirs, males less so ‘Mummy’s babies, daddy’s maybes’ ‘Mummy’s babies, daddy’s maybes’

Infidelity & Parental Investment Misattributed paternity prevalence is difficult to measure. Sample estimates: Anderson (2005) meta-analysis, studies of general population: 3.9% Anderson (2005) meta-analysis, studies of general population: 3.9% Bellis et al (2005) meta-analysis, studies of general population: 3.7% Bellis et al (2005) meta-analysis, studies of general population: 3.7% CSA (2005) cases of disputed paternity only: 16% CSA (2005) cases of disputed paternity only: 16%

Infidelity & Parental Investment

Infidelity & Parental Investment Because females can always be certain that the offspring carry their genes they are likely to invest more resources This tendency is multiplied across generations, so mother’s mother (guaranteed relationship) invests more than father’s father This tendency is multiplied across generations, so mother’s mother (guaranteed relationship) invests more than father’s father