While romantic jealousy has historically been viewed in pathological terms, evolutionary psychologists emphasize the utility of romantic jealousy as a basic adaptive mechanism designed to protect the pair-bond and, ultimately, promote reproductive success (Buss, 2000). One influential evolutionary hypothesis states that on account of parental investment theory (Trivers, 1972), males and females evolved distinct jealousy systems as a result of different reproductive challenges (Buss, Larsen, Westen, & Semmelroth, 1992). The “jealousy as a specific innate module” (JSIM) hypothesis (following Harris, 2003) states that males are more sensitive than females to cues of sexual infidelity to prevent cuckoldry, while females are more sensitive than males to cues of emotional infidelity to prevent loss of parental investment. Support for JSIM has been demonstrated at the perceptual and early cognitive level, and in behaviours evoked by jealousy such as mate-guarding and reactions to self-esteem threat (Goldenberg et al., 2003, Kaighobadi et al., 2010, Schutzwohl, 2005 and Schutzwohl and Koch, 2004). Furthermore, functional brain activity measured in response to scenarios of emotional versus sexual infidelity suggests that males and females have distinct and separate neuropsychological responses to each infidelity type (Takahashi et al., 2006).
The cornerstone of empirical evidence for JSIM, however, has rested on a dichotomous forced-choice task in which participants choose between sexual or emotional hypothetical infidelity scenarios as to the most distressing. While studies using the forced-choice task have demonstrated significant sex differences in line with JSIM, (Harris, 2003 and Madran, 2008), the wide variance in the percentage of males choosing sexual infidelity as more distressing compared with emotional infidelity has been highlighted: In two meta-analyses the percentage ranged from 12% to 76%, with a mean of 46% (Harris, 2003 and Madran, 2008). Such choice tasks seem subject to cultural variation and individual differences within sexes (Berman and Frazier, 2005 and Murphy et al., 2006).
An alternative to JSIM is the social-cognitive account of jealousy (Harris, 2003) which proposes that the long period of human infancy dependence selected for universal rather than sexually-specific mechanisms of jealousy in males and females (see Miller & Fishkin, 1997, for a full account). Accordingly, rather than being an adaptation, any between-sex differences in jealousy occur as a result of proximal mediators, such as threats to the self concept (which differs between- and within-sexes), the influence of cultural norms and diverse sex roles (Hupka, 1991, Salovey and Rothman, 1991 and Wood and Eagly, 2002).
The current study sought to identify a biological mechanism that would have been shaped by natural selection and, as expressed through proximate systems, could explain both between- and within-sex differences in romantic jealousy. Such a mechanism might lend support to the influence of natural selection on romantic jealousy specifically rather than to the idea of more general learning systems proposed by the social-cognitive account. We conjectured that a biological mechanism resulting in a “masculinised” brain might result in greater distress at sexual rather than emotional infidelity (“sexual jealousy”) in both sexes. Given the Takahashi et al. (2006) findings we proposed that such a mechanism could be the male sex hormone, testosterone. Indeed, comparative research supports the status of testosterone as a distal genetic force associated with pair-bond threat as it has been correlated with aggressive behavioural responses to pair-bond threat in macaque males (Rilling, Winslow, & Kilts, 2004).
Testosterone has two direct effects on the brain and behaviour: It has an organisational effect that causes permanent changes in brain structure in utero, specifically affecting the hypothalamus, the hippocampus and the amygdale ( McCarthy, 2010) and an activational effect post-puberty when circulating testosterone dynamically maintains the prenatally-determined sexually-dimorphic brain structure throughout adulthood ( Ahmed et al., 2008).
Herein we measured a putative indicator of prenatal testosterone, the ratio between the second and fourth digit (2D:4D), and examined its association with sexual jealousy. This ratio is lower in males than females, allegedly on account of the difference in prenatal testosterone exposure (Manning, Scutt, Wilson, & Lewis-Jones, 1998), and has been linked to specific behavioural traits in humans and non-human primates in a wide-ranging literature (Nelson and Shultz, 2010 and Voracek and Loibl, 2009). Behavioural associations with lower 2D:4D generally reflect sexually-selected, masculinised psychological capacities, for instance, promiscuity, conflict-related behaviour, adventure- and sensation-seeking have been reported in females (Clark, 2004, Hampson et al., 2008 and Voracek and Schicker, 2010) while aggression, sporting competitiveness and dominance have been reported in males (Bailey and Hurd, 2005b, Manning and Taylor, 2001 and Neave et al., 2003).
We predicted that if between-sex differences in romantic jealousy have been influenced by natural selection in accord with JSIM, low 2D:4D (i.e. a masculinised brain structure) would be negatively associated with sexual jealousy.
We have reported a small but significant association between 2D:4D, a correlate of prenatal testosterone, and sexual jealousy. Since underlying differences in brain structure on account of prenatal testosterone represent a distal genetic force, we have put forward the view that its association with between- and within-sex differences in romantic jealousy, i.e. whether sexual infidelity is more or less distressing than emotional infidelity, lends supports to JSIM. Accordingly, we discussed how a sexually-dimorphic brain structure could facilitate behavioural capacities in romantic jealousy from early childhood temperament to post-pubertal sexual desire. In sum, while an association between underlying differences in brain structure and sexual jealousy lends support to an adaptationist viewpoint, the weak relationship reported illustrates the complexity of the path from genes to behaviour and a full account of between- and within-sex differences in romantic jealousy requires a consideration of proximal factors. Accordingly, neither JSIM nor the social-cognitive perspective are mutually exclusive accounts.
