1. Behaviour as a product of evolutionary processes
Twentieth-century psychologists devoted considerable attention to the hypothesis that behaviour is learnt, and tended to downplay, or even completely deny, the possibility that behaviour has a heritable basis (e.g., Watson Reference Watson1919). A plethora of evidence, from across mammalian species, challenges this viewpoint. For example, rodents innately display species-typical defensive behaviour when exposed to a predator for the first time (Blanchard Reference Blanchard, Bouton and Fanselow1997); and, in humans, studies of twins have revealed that a wide range of behavioural traits are substantially heritable (e.g., Plomin et al. Reference Plomin, DeFries, McClearn and McGuffin2001).
One by-product of the emerging consensus that mammalian behaviour patterns are partly innate is a growing interest in explanations as to the mechanism by which behaviour becomes coded in the genome. By far the most prominent theory is that innate behaviours, such as aggression, evolved by similar processes to those that are widely accepted to have shaped anatomical features (Darwin Reference Darwin1859/1911). Following Darwin, Archer presents a detailed argument that human sex differences in aggression evolved primarily as a result of selection pressure that placed less aggressive males at a competitive disadvantage in the struggle for mating opportunities.
Archer is careful to note, however, that the greater propensity to aggressive behaviour in males is limited to attacks on male competitors. In situations where violence occurs between heterosexual couples or in situations of provocation, Archer cites studies that show males and females display similar levels of aggression (e.g., Archer Reference Archer2000a; Bettencourt & Kernahan Reference Bettencourt and Kernahan1997). We argue that Archer's already convincing argument could be further strengthened by supplementation with certain principles drawn from the work of Jeffrey Gray on defensive/predatory behaviours and personality (Gray & McNaughton Reference Gray and McNaughton2000; updated by McNaughton & Corr Reference McNaughton and Corr2004; Reference McNaughton, Corr and Corr2008; for a summary, see Corr Reference Corr and Corr2008).
2. Reinforcing stimuli as mediators of aggression
Gray's approach postulates that the various classes of emotional behaviour displayed by animals are dependent upon the interplay of three major neuropsychological systems. Threat stimuli activate a Fight-Flight-Freeze-System (FFFS), leading to avoidance/escape and the emotions of fear and panic/rage (depending on the level of threat perceived) – this system is specifically related to defensive aggression. Reward stimuli activate a Behavioural Approach System (BAS), leading to approach behaviour and the emotion of hopeful anticipation, optimism, and so forth – this system may be related to predatory aggression (see below). Ambiguous and conflicting stimuli (e.g., approach-avoidance conflict) activate a Behavioural Inhibition System (BIS), leading to risk assessing cautious behaviour and the emotion of anxiety (reflecting the apprehension that something bad may occur at any moment) – activation of this system should be expected to inhibit aggression. Applied to personality, this theory maintains that major traits (e.g., extraversion and neuroticism) originate in individual differences in the sensitivity of these brain systems to their adequately eliciting stimuli.
This theoretical perspective suggests that Archer's theme – that competitive aggression is predominantly a male phenomenon, whereas provoked aggression is equally common in males and females – may be explained by sex differences in functioning of these brain systems. For example, a male who assaults another male with the intention of improving his own mating prospects is displaying appetitively motivated predatory aggression (in this case, by the prospect of sexual intercourse) that is hypothetically mediated by the BAS. In support of this view, there is now mounting evidence that anger/aggression is, at least in part, elicited by BAS activity (Carver Reference Carver2004; Carver & Harmon-Jones Reference Carver and Harmon-Jones2009; Corr Reference Corr2002). The defensive-versus-predatory and negative-versus-positive distinctions receive additional support from findings that rats seek out opportunities to perpetrate predatory aggression – as if it were appetitive – yet they try to avoid perpetrating defensive aggression – as if it were unpleasant (Panksepp Reference Panksepp1971).
If, over time, predatory aggression boosts male fecundity, then this trait would spread throughout the male population by standard evolutionary means. As explained by Archer, the mechanics of mammalian reproduction mean that women face little or no selection pressure to harm or kill female rivals, and so a trait for predatory aggression should not be expected to proliferate in the female population. Given that individual variations are observed in the functioning of the FFFS, BAS, and BIS, personality differences, both within and between the sexes, should be expected in defensive and predatory aggression. For example, a personality dimension, such as impulsive-unsocialised-sensation seeking (often labelled “psychoticism”; Eysenck & Eysenck Reference Eysenck and Eysenck1991) would be a promising candidate for a measure of predatory tendencies. This idea is supported by males typically scoring higher on these traits (e.g., Diaz & Pickering Reference Diaz and Pickering1993).
With regard to defensive aggression, an evolutionary view, such as that advanced by Archer, would predict there should be little significant difference between the sexes, as both sexes face selection pressure to defend themselves against aggressive conspecifics and other species. This idea is supported by evidence that females are as likely as males to be aggressive when provoked (Bettencourt & Kernahan Reference Bettencourt and Kernahan1997), which is further supported by study of human defence reactions using a threat scenario questionnaire approach (Blanchard et al. Reference Blanchard, Hynd, Minke, Minemoto and Blanchard2001). This questionnaire requires participants to select their preferred defensive response to each of 12 threat scenarios from a list of 10 defensive options (e.g., fight, run away). Typically, preferred defensive reactions to scenarios describing especially close or inescapable threats consist of explosive behaviour and aggression (e.g., yell, scream, attack, struggle), and this is equally likely for men and women (e.g., Perkins & Corr Reference Perkins and Corr2006).
If the distinction between defensive and predatory aggression is valid, then a dissociation should exist in human preferences for exhibiting aggression, with both sexes seeking to avoid situations that require defensive aggression, but males (especially those who score high on traits such as psychoticism) preferentially seeking opportunities to perpetrate predatory aggression. This hypothesis has yet to be tested in humans, but it is supported anecdotally by the predominantly male enjoyment of, and participation in, rough or violent sports such as boxing, rugby, and American football. It would be valuable to know the importance Archer attaches to the defensive-versus-predatory distinction, and the role played by personality factors in aggressive behaviours; we would be interested in learning about the possible evolutionary pressures on the variation of aggression seen within the sexes.
