There are numerous factors that impact human behaviour, and both biological and psychological causes are important in understanding it.
The role of hormones in controlling behavior, in particular aggression, has been widely studied by psychologists for many years, but without any definite conclusion.
Defining Aggression and Hormones:
Aggression is behaviour which causes intentional harm to another person (Anderson, 2002). More specifically, aggression is defined as any sequence of behavior, the goal response to which is the injury of the person toward whom it is directed (Dollard et al.,1939).
Although most psychologists agree that it is the actual observable behaviour causing harm that defines aggression, there are definitions that simply imply threat or intimidation, and include behaviour like biting, goring, butting, hitting, play fighting, rough and tumble play. (Bland, 2004). However in biological studies, it means the outcome of direct competition when resources are limited, and where species must compete with one another in order to increase their own fitness (Bland, 2004).
Hormones are chemicals, carried along the bloodstream to other parts of the body, where they act on target tissue to produce physiological effects (Rosenzweig et el, 1999). They are linked to behaviour because of the impact their presence has on an organism, and though it’s not their primary function, neural activity can alter because of them.
Hormones can also modify cell permeability and have significant effects on ion concentration, membrane potential, synaptic transmission, neural communication and hence, behaviour (Simpson, 2001).
Role of Hormones on aggression:
It is believed that high levels of hormones, testosterone in particular, are necessary though not sufficient to trigger aggressive behavior. Testosterone is the primary androgen, a class of steroid hormones that is secreted into the male testis, female ovaries, and in small amounts by the adrenal glands of both sexes.
According to some studies, there is a critical period, shortly after birth, when testosterone sensitizes certain neural pathways in the brain, and when these circuits are stimulated again by steroids in adulthood, aggressive behavior is elicited. (Dixson, 1993).
Testosterone acts as a pro-hormone which when converted into 5-alpha-dihydrotestosterone (5α-DHT) acts on androgen receptors or when converted into estradiol by the enzyme aromatase, acts on estrogen receptors (Figure 1).
Also experimental data shows that when testosterone acts on synapses it lowers the amount of neurotransmitter 5-HT available for synaptic transmission, significant because the presence of 5-HT serves to inhibit aggression (Simpson, 2001).
Studies and Experiments to show link between Hormones and Aggression:
Human studies have so far shown a relationship between testosterone in males and aggression (Olweus el et, 1988). Since males secrete more testosterone than females and appear to engage in more frequent fighting, not enough studies have been done on female aggression. Also, there are differences in the effects induced by testosterone between males and females, as different regions in the brain modulate hormone-dependent aggression (Albert el et, 1993). It is believed that testosterone leads to the establishment of an "androgen-responsive system" in males, while a similar androgen system is set-up in females, except greater exposure to androgens is required to induce male-like fighting (Simpson, 2001).
Nick Neave (2008) examines the relationships between hormones and behaviour in both humans and animals and demonstrates ways in which hormones can influence brain structure and function, including specific behaviours like aggression.
Interviews, questionnaires and criminal records show a positive correlation between violence in prisoners and hormones, though it is unclear whether hormones facilitate aggression or simply encourage dominance, competitiveness, and impulsiveness (Nelson, 1995). Dabbs et al. (1991) found prisoners high in testosterone committed more violent crimes, were judged more harshly by the parole board, and violated prison rules more often than those low in testosterone.
Kreuz and Rose (1972) however, found no significant testosterone difference between those who fought more in prison and those who did not, though prisoners with a prior record of aggressive crimes had significantly higher testosterone than those without such a history.
Scaramella and Brown (1978), studying hockey players, found a significant correlation between testosterone and coach ratings of players' aggressiveness in response to threat. Lindman et al. (1987) found significantly higher testosterone among those judged by their peers to be most aggressive while drunk. Harrison et al (2000) found that testosterone treatment significantly increased aggressive responses on a frustration-inducing computer games, though this effect was not uniform across individuals.
In women, there is greater variance in hormones affecting aggressive beahviour. Ehlers et al. (1980), studying women patients in a neurological clinic, found significantly higher testosterone among aggressive patients compared to less aggressive ones, but these groups also differed in diagnosis, making the comparison suspect.
Dabbs et al. (1988) saw no difference in testosterone between female prisoners and college students, but women convicted of unprovoked violence had higher testosterone than other prisoners. Gladue (1991) found testosterone to be negatively related to self reported aggression in women.
However, in one study where hormones (total testosterone, androstenedione, estradiol, and cortisol) were measured, results indicate that women with low levels of androstenedione and total testosterone were less likely to express their competitive feelings overtly, while women with high levels of androstenedione were more likely than other women to express their competitive feelings through verbal aggression (Cashdan, 2003).
Experiments On Animals:
Numerous experiments on rodents confirm that raising testosterone increases aggressiveness (Svare, 1983; Monaghan and Glickman, 1992). In one experiment on mice, it is seen that castration leads to a marked decrease in aggression.
However, when they were given testosterone replacement therapy, only mice initially rated as aggressive showed a restoration of aggressive behaviour, showing that injecting testosterone is not sufficient to turn a previously non-aggressive mouse into an aggressive one (Van de Poll et al, 1988). More recently it is seen that aggressive behaviour in rats induced by electrical stimulation of the hypothalamus was enhanced by androgens (Bermond et al, 1982).
In one study, ovariectomized female rats were given daily injections of testosterone, estradiol or a placebo. It was found that testosterone increased aggressiveness, measured by the frequency of fighting, whereas estradiol or a placebo had little effect (Van de Poll NE el at 1988).
In general, female aggression patterns were similar to those seen in males, i.e. ovariectomy without hormone replacement decreased fighting and was stimulated by replacement of testosterone (Albert el et, 1993).
One of the difficulties in the study of a relationship between hormones and aggression, is the definition of the term ‘aggression’ itself. Since a range of behaviour can fall under it, it is not easy to search for a common physiological mechanism on which hormones might act (Donovan, 1985).
It is seen that testosterone does not directly affect human aggression, as in intentional infliction of injury, but high levels of it do encourage dominant behaviour intended to achieve or maintain high status (Mazur, 1997).
While most studies have focused on males and testosterone, other hormones like adrenocorticotropic, prolactin, estrogen, progesterone, and adrenalin also affect aggressive behaviour. It is important to remember that the endocrine system consists of a complex array of communication pathways, none of which act independently (Simpson, 2001).
Also, hormones cannot cause a particular behaviour; they can only encourage or inhibit behavior by sensitizing receptors for a potential outcome. McEwen (1994) too confirms that hormones are not the direct cause of behaviour, but induce chemical changes in certain neurons, which affect the likelihood of certain behavioral outcomes.
For example, the mere presence or level of testosterone is not sufficient in causing aggression, as seen by a significant population of males that are not aggressive. It is obvious that others factors including cognition, social and environmental influences affect the expression of aggressive behaviour (Thiessen 1976).
Other studies show that it is not testosterone level but obesity and lower levels of "good" cholesterol that is the best predictor of aggression in human males (DeNoon, 2003).
Moreover, behavioral effects of hormones are not uniform across individuals and variations are seen because of timing of exposure, the organism's sensitivity to the hormones, the specific hormone involved, and modification by the physical and social environment, although not all factors have been studied in all species, and many have not been studied directly in people (Berenbaum, 1998).
Also, because of practical and ethical limitations in observing or even allowing high aggression in human subjects, researchers often measure aggression through questionnaires and paper-pencil tests and performance on these tests is often subjective and not always correlated with actual aggressive acts (Bagatell el et, 1994).
Scientists are limited because they cannot perform experiments on humans as they have done on animals and since behaviour patterns in humans are so different from animals, results from animal studies cannot be generalised.
Even though there is considerable evidence that hormones are correlated with aggressive behavior, correlation does not mean causation. Before experiments can be done to investigate these correlations, we need to be sure that the types of aggression we are looking into are testosterone-dependent and eliminate other factors that can confound results.
The lack of consistency among these correlational findings means that further research is required. Lastly, we must not limit our scope to biological factors, nor underestimate the role of social and environmental factors for a better understanding of the role that hormones play in aggressive behaviour.