Hormones are chemical messengers released into the bloodstream that travel through the body to influence distant organs and, importantly, the brain. Because they act widely and slowly compared with the rapid signals of nerves, hormones shape behaviour less like a switch and more like a tide, raising or lowering the likelihood of certain responses. The study of hormones and behaviour asks how these messengers and the nervous system shape one another in a continual two way conversation.
A central insight is that the relationship between hormones and behaviour runs in both directions. Hormones can change how an animal behaves, making aggression, care or courtship more likely, but behaviour and experience in turn change the levels of hormones. This reciprocal influence means that hormones rarely dictate behaviour outright; instead they adjust the probability of responses within a particular context and history.
What Hormones Are and How They Act
Hormones are produced by glands and released into the blood, where they reach tissues throughout the body and bind only to cells equipped with the right receptors. This lock and key arrangement allows a single hormone to produce different effects in different tissues, and it explains why hormonal influence is widespread yet selective. Compared with the swift, pinpoint signals of the nervous system, hormonal signals are slower and more diffuse, which suits them to shaping sustained states such as readiness to mate, to defend or to nurture.
The Two Way Street Between Hormone and Behaviour
It is tempting to think that hormones simply cause behaviours, but the influence is reciprocal. A rise in a given hormone may make a behaviour more likely, yet engaging in that behaviour, or merely encountering a relevant situation, can itself raise or lower hormone levels. Winning or losing a contest, caring for an infant, or facing a threat can all shift the very hormones that shape such responses, so hormone and behaviour continually adjust one another.
Organising and Activating Effects
Hormones influence behaviour in two broad ways across the lifespan. Early in development they can have organising effects, shaping the structure of the body and brain in lasting ways during sensitive periods. Later they have activating effects, temporarily switching on or heightening behaviours in a body already prepared, as when seasonal changes in hormones bring on courtship or territorial behaviour. The same behaviour may thus depend both on early organisation and on later activation.
Context, Receptors and the Limits of Hormonal Explanation
Because a hormone acts only where receptors receive it, and because its effects depend on the situation and the individual, hormonal explanations of behaviour must be made with care. The same hormone can promote one response in one context and a different response in another, and levels alone predict little without knowing the receptors, the history and the circumstances. Recognising these limits guards against the simple and mistaken idea that a single hormone straightforwardly produces a single behaviour.