In two large surveys (with a total of over 37,000 responders), conducted in France and Belgium, Quoidbach et al. show that
emodiversity is an independent predictor of mental and physical health – such as decreased depression and doctor’s visits – over and above mean levels of positive and negative emotion.
They defined emodiversity as follows: the survey asked participants to rate how often they experienced each of 9 positive and 9 negative emotions (or 10 in the second study). Each emotion was rated on a 5 point scale of 0 = never and 4 = most of the time. Emodiversity was calculated using this formula
What this means is that someone who reported only ever feeling one positive emotion would have an emodiversity of 0, while someone who endorsed all 9 positive emotions equally would get the maximum score. Emodiversity for negative emotions was calculated a similar way, while 'overall' emodiversity considered all 18 emotions. It turns out that emotional diversity was a good thing (in terms of being associated with less depression etc.) for both positive and for negative emotions. This seems a little counter-intuitive. You might have expected that feeling many negative emotions would be worse than only feeling one of them - but in fact, it's better. Why is this? The authors speculate that
[just as] biodiversity increases resilience to negative events because a single predator cannot wipe out an entire ecosystem, emodiversity may prevent specific emotions – in particular detrimental ones such as acute stress, anger or sadness – from dominating the emotional ecosystem. For instance, the experience of prolonged sadness might lead to depression but the joint experience of sadness and anger – although unpleasant – might prevent individuals from completely withdrawing from their environment. The same biodiversity analogy could be applied to positive emotion. Humans are notoriously quick to adapt to repeated exposure to a given positive emotional experience; positive experiences that are diverse may be more resistant to such extinction.
This makes sense - though it assumes that emodiversity is the cause of being healthy and not depressed. As this study was cross-sectional and correlational, it can't tell us that. It might be that one of the effects of depression (say) is a reduced range of emotions. Quoidbach et al. do note this possibility, but they clearly favor the idea that emodiversity is the cause of well-being, even claiming in the abstract to have shown "the benefits of emodiversity" which sounds rather like the (sub)title of an upmarket self-help book. Still, there's no denying that this is a huge study and that the correlation - an interesting one - is real enough.
Quoidbach J, Gruber J, Mikolajczak M, Kogan A, Kotsou I, & Norton MI (2014). Emodiversity and the Emotional Ecosystem. Journal of Experimental Psychology. General PMID: 25285428