Captivity often restricts the abilities of animals to perform natural behaviour. Here, we review how this constraint affects psychological welfare by preventing the meeting of motivations. One means by which this happens is through frustrating specific motivations pertaining to particular behavioural systems. This can occur when constrained behaviours are 'behavioural needs': activities that animals have instincts to perform even in environments where they are not biologically necessary for fitness (e.g. non-nutritive sucking by calves). It can also occur when deficits or external cues in the environment elicit strong motivations to behave a certain way (e.g. the lack of burrow-like structures triggering digging attempts in gerbils). Furthermore, given that humans suffer boredom in monotonous conditions that resemble those of the environments of many captive animals, and that many animals actively seek stimulation, it seems likely that, at least for some individuals in some species, behavioural restriction also harms welfare by thwarting general motivations to seek variety and/or to avoid monotony, thus causing boredom.