Behavioural endocrinology is the study of the interaction between hormones and behaviour. This interaction is bidirectional: hormones can affect behaviour, and behaviour can alter hormone levels. Thus, hormonal-behavioural correlations can be due to hormonal effects on behaviour, but certain behaviour (such as physical exercise, stress, sexual behaviour, alcohol consumption, and nutrition) is known to influence hormone levels as well (see below and Christiansen 1999).

Hormones do not cause behavioural changes per se; they can only alter the probability that particular behaviour will occur in the presence of a particular stimulus. Hormones can influence regions of the central nervous system (CNS) which contain hormone receptors by inducing changes in the rate of cellular function. The interaction of a hormone with its receptor begins a series of cellular events that lead to a genomic response wherein the hormone acts directly or indirectly to activate genes that regulate protein synthesis (e.g., Bixo et al. 1995; Chalepakis et al. 1990; Ford and Cramer 1982; Genazzani etal. 1992; Hutchinson 1991; McEwen 1992; McEwen etal. 1984; Sekeris 1990; Viru 1991).

Two decisive phases have been named in the discussion about the time of the effects of sex hormones on brain structures and consequently on behaviour. During fetal and neonatal life, relatively high concentrations of hormones, especially testosterone, are said to influence brain development by organizing the undifferentiated brain in a sex-specific manner. It has been shown, according to studies primarily in rodents, but also in primates and other mammals, that the hypothalamus, the hippocampus, the preoptic-septal region, and the limbic system (especially the amygdala) are important target areas for sex steroid action (Bettini etal. 1992; Brain and Haug 1992; Collaerand Hines 1995; Ellis 1982; Ford and Cramer 1982; Hutchinson 1991; 1993; Hutchinson and Steimer 1984; McEwen 1992; Michael and Bonsall 1990; Naftolin et al. 1990; Simon and Whalen 1987; Whalen 1982). These brain structures and hence the corresponding behavioural repertoires are then thought to be activated at the beginning of puberty when the production of sex hormones increases (Archer 1988; Beatty 1979; Becker etal. 1992; Schulkin 1993). However, experimental studies of birds, rodents, and monkeys have demonstrated that an animal's previous experiences in aggressive encounters can sometimes be more important than the testosterone level in determining an individual's aggressiveness or dominance (Archer 1991; Gordon etal. 1979; Rejeski etal. 1988).

In humans, hormonal influences on behaviour are much less potent than in animals. Quantitative and qualitative behavioural differences in males and females are thought to result mainly from a combination of psychosocial factors which are the end product of differential experience and expectation produced by socialization. Moreover, hormonal influences on human behaviour are difficult to prove as pertinent research has to rely predominantly on correlational studies of endogenous hormone levels and behaviour which cannot ascertain hormonal influences. Some knowledge derives from clinical studies on individuals who have been exposed to atypical levels of hormones during some developmental period of their lives, so-called "experiments of nature". Only on very few occasions can scientists ethically manipulate hormone levels in humans to observe subsequent effects on brain and behaviour. But even from these studies on hormone substitution one cannot always draw firm conclusions regarding a particular hormonal-behavioural relation. If any, results from double-blind, placebo-controlled experimental designs can be interpreted as meaning that a particular hormone is the metabolic agent associated with behaviour. However, in behavioural endocrinology of humans, these are rare exceptions. Therefore, conclusions regarding hormonal effects on human behaviour have to be drawn with great care.

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