Stereotypic, or stereotypical, behaviour is the collective term of very diverse responses towards aversible stimuli and the causal factors behind are very diverse too.

* The Form these stereotypies can take on depends of the specie in question. It is e.g. highly unlikely that an oral-inclined pig will ever be able to engage in the same type of backward somersaulting as the far more acrobatic mink.

* Within a given specie there can be large differences on the individual level - a phenomenon, which has achieved increased attention since the mid-eighties.

* These individual differences, or variations, can be caused by factors coming from within the physiology of the individual and physiology equals complexity, e.g. regarding the various neurotransmitters and -receptors. At present, a large number of different neurotransmitters has been determined, systems which probably all interact with each other and with the different glandular systems. It is highly likely that new neurotransmitters will be determined in the future and that the already established neurotransmitters eventual will be shown to have additional ways of interacting with each other.

* The individual state of nutrition can have an effect on the behaviour just as the genetic make-up is likely to have an effect regarding predispositions towards development of stereotypies in captivity or during pharmacologic experiments.

* Another element of significance when it comes to stereotypes is the individual state of previous learning. This factor is often more or less unknown, often overlooked, but should be taken into account

The apparent purposelessness of stereotypic behaviours has caused a large number of hypotheses of which only a handful have been mentioned. The ones which have been touched upon has hypothesized that stereotypies serve to up- or down regulate Arousal, or maybe serve a homeostatic function regarding Arousal; that the behaviour makes the individual cope better with aversible stimuli and by that changing the perception of these in a less harmful direction; that stereotypies stems from boredom and is a way for the individual to maintain a proper sense of time/meaning; that the stereotypies is rewarding in them selves or that they are a result of genetic dispositions (see here and here). Finally, it cannot be ruled out that stereotypies simply are a symptom, or a pathological side effect, of a disturbed transmitter function, and the cause of such could be the topic of a book.

At the time these chapters is written (1998) there is no uniting theory, which can explain the purpose of stereotypic behaviours, and such a theory is not within immediate reach. Any monocausality is not to be expected due to the heterogeneity of these stereotypies, but off course, it does not help particularly for the progress of science that the ratio of reviews to experimental papers within this special field is roughly 1: 3.

***Note (2006): There is still no grand uniting theory for stereotypic behaviour in sight, but there has been a certain development within the field. The ratio of reviews to experimental papers has improved marginally too - now it's 1: 3.18***