Back in the 1960s, Hans Eysenck and Sybil Eysenck conducted an experiment that suggested that introverts (identified through a questionnaire) salivated more than extraverts when exposed to lemon juice, presumably because they have a higher baseline level of cortical arousal. The results were widely popularized; in an interview with Scientific American, Susan Cain said, “Introverts even salivate more than extroverts do if you place a drop of lemon juice on their tongues!” If you Google “lemon introvert salivation” you will see thousands of mentions of the study and minimal critical discussion.
Now, this study has problems; later studies called its findings into question. (What nerve! I think of Andrew Gelman’s “Enough with the Replication Police.”) I intend to dust off my own police uniform and look into all of this. For now, I will focus on the error that comes up again and again in interpretations of this test. People now claim that you can find out how introverted you are by conducting the lemon experiment. That is not only preposterous but illogical.
It is one thing to say that a study suggests that introverts tend to salivate more than extraverts in response to lemon juice. I question such a claim and the rigor of the study that led to it, but that’s what you’re supposed to do with such studies anyway. Now, to claim the reverse—that you can find out how much of an introvert you are by putting lemon on your tongue and measuring your saliva output—is to succumb to the famous fallacy of affirming the consequent.
Here’s why it’s wrong. Studies like the lemon juice experiment draw general conclusions from an array of individual results. Within the experiment, there may have been introverts who salivated less than extraverts. There may have been quite a few introverts and extraverts who salivated at similar levels. It might even be the case that if you divided salivation levels into two groups, a “low salivation group” and a “high salivation group,” you would find comparable numbers of introverts and extraverts in each. In no way does the test even suggest that if you salivate a lot, then you are an introvert.
Who is claiming such a thing, anyway? The BBC declares, “The amount of saliva you produce after putting a drop of lemon juice on your tongue might tell you something about your personality.” (Shame on them!) But that article has no listed author; it’s possible an intern wrote it. I give the BBC the benefit of the doubt it failed to cast on itself.
I see no excuse, though, for the famous TED-talking scholar Brian Little, who writes in Me, Myself, and Us: The Science of Personality and the Art of Well-Being (2014):
One of the more interesting ways of informally assessing extraversion at the biogenic level is to do the lemon-drop test. [Description of experiment omitted from present quote—DS.] For some people the swab will remain horizontal. For others it will dip on the lemon juice end. Can you guess which? For the extraverts, the swab stays relatively horizontal, but for introverts it dips. … I have done this exercise on myself a number of times, and each time my swab dips deeply. I am, at least by this measure, a biogenic introvert.
Someone of Little’s stature and renown should exercise more responsibility. He not only generalizes “introverts” and “extraverts” but suggests that you can conduct this experiment on yourself and find out who you are. The media (with exceptions) drools over this sort of thing; perhaps bad reasoning is a lemon, and perhaps the press as a whole has high cortical arousal.