Sadly, it's far from clear that he's right. Decades of research into the relationship between reasoning ability and well-being have failed to find a clear link. But now comes a ray of hope for high-quality thinkers--a study suggesting that Marcus Aurelius is right so long as you define "quality of thought" carefully. And the study comes with a good pedigree--it will be published in the prestigious Journal of Experimental Psychology and features the eminent psychologist Richard Nisbett among its co-authors.
What's correlated with well-being, say Nisbett, Igor Grossman, and three other authors, isn't reasoning ability in the abstract but rather "wise reasoning"--reasoning that is "pragmatic," helping us "navigate important challenges in social life."
So, for starters, how did the researchers measure wise reasoning? Subjects in this study read a series of accounts of social conflicts and Dear-Abby-like dilemmas and then, in oral interviews, were invited to discuss how the stories might unfold in the future. Their responses were rated along such dimensions as "considering the perspectives of people involved in the conflict," "recognizing uncertainty and the limits of knowledge," and "recognizing the importance of ... compromise between opposing viewpoints." These ratings were the basis for a "wise reasoning" score.For each of the subjects a second score was calculated that was intended to measure well-being. Its components included reported satisfaction with their lives and with their social relationships and a tendency toward positive expression.
It turned out that the two scores were correlated: the wiser people were, the higher their well-being.
Three interesting wrinkles:
 The older you get, the stronger the correlation. Wise young adults didn't exhibit much higher well-being than unwise young adults, but wise senior citizens had considerably higher well-being than their unwise peers. (Compare the slopes of the lines in the graph above.) So if you're young, cultivating wisdom is mainly a long-term investment. (That's probably a weak sales pitch for wisdom, since young people aren't known for thinking long term. I'm tempted to say they lack the wisdom to seek wisdom, but that would mean departing from this study's definition of wisdom, so never mind.)
 A second age-related issue: Well-being increases with age, and so does wise reasoning. Is it possible that getting older increases well-being and wisdom independently--that the wisdom itself has no effect on well-being? After all, gray hair increases with age and so does joint stiffness, but gray hair doesn't cause joint stiffness.
Through a statistical technique that I don't claim to grasp, the authors conclude that the answer is mixed. Part of the increase in well-being associated with age is caused by growing wisdom, but part of the increase happens for some other reason. That is, wisdom, is a "partially mediating" variable between age and well-being.
 Another causality question: Leaving aside the age issue, how should we interpret the general correlation between wise reasoning and well-being? Assuming a causal link between these two variables, does the wisdom lead to the well-being or does the well-being lead to the wisdom?
The latter is certainly plausible. When I'm in a good mood, it's easier to consider the perspectives of other people, and easier to focus on compromise--two components of wisdom as defined here. And presumably if I were in a good mood more often--if I had an enduringly high sense of well-being--my ability to thus exercise wisdom would remain pretty high.
The authors consider this question and offer grounds for doubting that it's the well-being that causes the wisdom, but they concede that the issue isn't completely settled.
I'm guessing the answer is a little of both: Wisdom leads to well-being, and well-being paves the way for wisdom--and, in particular, for wise action, not just a capacity for wise reasoning.
If that's true, then you can imagine getting swept up in a virtuous circle: Acting wisely reduces conflict in your life and strengthens your social relationships, and this fosters a sense of well-being that makes it easier to act wisely, and so on. But there's also the vicious circle scenario--a downward spiral featuring growing unhappiness, commensurately unwise action, deeper unhappiness, and so on.
The virtuous circle scenario is certainly more appealing. And it sounds like it wouldn't be that hard. But I'm old enough to know better.
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