The process of behaviour ontogeny occurs epigenetically, i.e. from the interaction at any one time between phenotype, active genes and the environment within the range of the phylogenetically formed "Reaktionsnorm". Particularly during the post-embryonic stage, learning processes, such as habituation, classical and operant conditioning, imitation, perceptive learning and imprinting, will influence behaviour ontogeny. In view of the restrictive housing conditions of farm and laboratory animals and of neurophysiological findings it may be postulated etiologically speaking that abnormal behaviour should be attributed to a changed network and differentiation within the brain which may have occurred during early ontogeny (e.g. deprivation and missing or misdirected imprinting). In addition, behaviour disorders may result directly from a dysfunction at the level of information coordination. Animals strive without success to adapt to a rigid, unavoidable and artificial housing environment and as a result develop coping strategies which are maladaptive (e.g. stereotypies, learned helplessness). In order to prevent behaviour disorders, research into their origin is indispensable, as indeed is an examination of the ontogeny of normal behaviour. Only in this way can new housing systems be developed where in spite of restriction crucial stimuli may nevertheless be provided which will enable animals to develop and to breed without suffering harm.