There are a number of abnormal behaviours that are depicted in this text which is so much about the nature of mental illness. However, arguably, one of the main themes of this work is that what are termed "mental disorders" are actually a product of societal pressures and stresses that cause individuals to respond in a way that makes them classed as being "insane," whereas the state of being insane has more to do with behaving in a way that falls beyond the realms of what is deemed to be "normal" behaviour. A prime example of this is Bromden, who exhibits plenty of abnormal behaviour: he stays silent, he doesn't engage in communication and he believes he is small when in fact he is the tallest person at the asylum. However, what he comes to realise, through the tutelage of McMurphy, is that what others call "insanity" is only what happens when the pressures placed on individuals by society get too great. Note how he comes to this realisation on the fishing trip when McMurphy faces the chaos that occurs with laughter:
While McMurphy laughs. Rocking farther and farther backward against the cabin top, spreading his laugh out across the water—laughing at the girl, the guys, at George, at me sucking my bleeding thumb, at the captain back at the pier and the bicycle rider and the service-station guys and the five thousand houses and the Big Nurse and all of it. Because he knows you have to laugh at the things that hurt you just to keep yourself in balance, just to keep the world from running you plumb crazy.
Bromden here implies that it is society, depicted in the captain, the "five thousand houses" and the "Big Nurse" is what drives people insane. When people become overwhelmed by the pain and suffering in society it is difficult to not become "plumb crazy" and to "keep yourself in balance." Although this novel therefore features plenty of examples of abnormal behaviour, it argues very strongly that mental disorders are just a result of the pressures of society and individuals not being able to withstand those pressures. McMurphy teaches Bromden and the other inmates about the importance of meeting such pressures with laughter to counteract them and to stay sane.