Why Should Happiness Boost The Immune System?

There’s an apparent paradox in the research on positive emotions, and how they relate to the immune system, something that doesn’t make sense on first inspection. But like many things in psychology, I think the answer has something to do with sex.

I was quite surprised to find out how much research there is into the effects of positive mood on our physiological state. It’s interesting how many markers there are of positive emotions in our physiology. Some are quite obvious, such as duchenne smiles and other changes in facial expression, but there are also changes in neurochemistry, and activation in the left prefrontal cortex of the brain.

One particularly interesting finding, though, is the result that positive emotions evoke an increased immune response. One example of such a study forced the activation of the left side of the brain using transcranial magnetic stimulation (something which sounds awfully painful but is apparently non-intrusive). This process actually increased the concentration of immunoglobin A in the saliva. This effect does not appear to be specific to the use of magnetic stimulation in general, because although stimulating the right side of the brain also caused an increase in immunoglobin A, the concentration dropped as soon as the stimulation of the brain did, while it remained for some time after left-side activation. (1)

But the interesting question to me is, why would the immune system increase it’s output along with the experience of positive emotions? According to Barbara Fredrickson, positive emotions signal a time when things are going well, when there are no particular threats around. If there was a threat that we should attend to, the brain would narrow our thought-action repertoire towards dealing with that threat in the form of a ‘negative’ emotion. However, when there is no particular threat, when things are going well, we’re better off broadening our perceptions and action repertoires, because, the theory goes, this will allow us to build our personal and social resources in preparation for future challenges. This is broaden-and-build theory, it’s a theory behind the purpose of positive emotions (and probably deserves its own post to be honest).

At first glance broaden-and-build theory seems somewhat at odds with the immune system studies. Surely, a time of broadened perception – a threat-free time – is a time to let the immune system rest a little. With no perceived threat around, why mobilise our defences? Why waste the energy? And also, why reduce our immune response when their is a threat?

These findings suggest that there is something about the behaviours associated with positive and negative emotions that require higher and lower immune responses, respectively. Researchers apparently don’t know what that is yet, but there are some possibilities.

Positive emotions are associated with approach related behaviours. This might include exploring the environment, playing, and, crucially, spending time with other people. Perhaps this latter point is important. Parasites and viruses would themselves be evolved and specialised for transmission between humans, and since times of positive emotions might involve building relationships with other people, including physical closeness, and in particular, more sexual activity, this is the time that the immune system is needed most.

In other words, we might be more likely to catch an infection from another human than we are from the things in the physical environment that trigger a negative emotion in us. When we come across the proverbial sabre toothed tiger, catching a cold is the least of our worries. In these situations, we’re better off diverting our energy away from our immune system, towards our muscles, and getting the hell out of there (or fighting).

Unfortunately I’m just speculating here, I definitely could be wrong and the reasons for this effect are not yet fully understood. But it’s an interesting possibility.


Clow, A., Lambert, S., Evans., P., Hucklebridge, F., Higuchi, K. (2003). An investigation into asymmetrical cortical regulation of salivary S-IgA in conscious man using transcranial magnetic stimulation. International Journal of Psychopathology, 47, 57-64.

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