Joy and sadness are two very basic emotions, often thought of as complementary, as in the classical Greek theatre masks, one happy and smiling – comedy – and the other sad – tragedy. But they are not two sides of the same coin; they are the evolutionary result of two very different emotional strategies, both of which have emerged to guide our behaviour and inform our environment.
The so–called basic emotions, which we share with all other mammals – and probably even more animals – are there to provide a rapid and essentially automatic response to particular life situations, which can be shaped by learning and memory. In the most basic repertoire we have the responses of fear, anger, sadness, and happiness. All have their evolutionary function: fear keeps us away from certain dangers, and anger alerts us to a threat.
But what is the role of sadness and happiness? All the evidence suggests that sadness evolved as a response to the need for social contact, and that happiness functions as a reinforcement of social interactions. Following the research of Jaak Panksepp in particular, much progress has been made in understanding the neural mechanisms behind these two emotions, and we now have an idea of the threads we need to pull together to understand their evolutionary origins. It turns out that at the end of the evolutionary thread of joy we find something perhaps as frivolous in principle as physical play; and at the end of the thread of sadness we find panic or separation anxiety.
Baby mammals and other animals do not like to be left alone, they panic and start making distress calls. This is not because they want to annoy others, but because it is an effective evolutionary mechanism to increase the survival of these crybabies. Research shows that the neural circuits behind this behaviour are maintained throughout development and are the same ones that trigger the emotion of sadness in adults – the brain regions involved include the bed nucleus of the stria terminalis, the periaqueductal grey matter and the amygdala. Having an animal model to study sadness is important because abnormal or exacerbated functioning of these neural networks may underlie some forms of depression.
Happiness, on the other hand, seems to come from fighting. Not real fights, but the kind that young mammals simulate when they play. This is not a trivial activity, but part of an intensive programme of cognitive and motor training. In human offspring – and at least in chimpanzees and rats – this physical play is often accompanied by laughter, a curious vocalisation that is in most cases useful as an indicator of happiness. The idea that has emerged from studying the brains and behaviour of these mammals is that the neural circuits that drive us to play as children are the same ones that control the basic emotion of happiness in adulthood – including the parafascicular thalamic nucleus, the nucleus accumbens and, in primates, the insula.
Sadness and happiness may not be two opposing modes of operation in the same emotional system – they appear to have very different neural circuits and evolutionary origins – but their functions in human behaviour ultimately converge on a single purpose: to facilitate social contact and interaction. We are a hyper–social species whose evolutionary success is in large part due to comedy and tragedy.
