Do you find yourself salivating at the merest thought of eating a lemon? The answer may say more about your mind than your taste for sour flavours, as Christian Jarrett reveals in the first post of his new column Personology.

Do you think you’re an extravert or an introvert? To answer, you might immediately ponder how much you like going to parties and talking to strangers, or perhaps you’ve already got an answer given to you by one of those online quizzes that prompt such self-reflection. The trouble with these approaches of course is that they rely on honest insight and a hefty dose of subjectivity. Maybe you quite like parties, for instance, but nowhere near as much your best friend – does that make you an extravert or not?

A completely different approach that gets round these problems involves using a lemon – more specifically, concentrated lemon juice. This is a test with a long history in personality psychology and it’s really easy to try out at home. You’ll need a cotton bud (or what’s called a cotton swab or Q-tip in the US) with a short piece of thread tied exactly in the middle of it. Now place one end of the cotton bud on your tongue for 20 seconds. Next, put five drops of concentrated lemon juice onto your tongue, swallow, and then put the other end of the cotton bud onto your tongue for 20 seconds. Finally, take the end of the cotton bud out of your mouth and hold the cotton bud dangling by the thread. The idea is to see whether it hangs horizontally, or whether the end that you used after the lemon juice hangs lower because it is heavier.

If your reaction to the lemon juice made one end heavier this suggests that the juice caused you to salivate more than normal, which is a sign that – at a physiological level – you are an introvert. If the cotton bud is horizontal, this suggests you didn’t react much to the lemon juice and that you are probably an extravert. 

How come? This is a version of a test described way back in the 1960s by one of the pioneers of personality psychology, Hans Eysenck, and his wife and fellow personality researcher Sybil Eysenck. In the original experiment, they used sensitive weighing equipment to measure how much saliva was absorbed into a cotton bud before and after people were exposed to the juice. (The simplified DIY version described here comes from the contemporary personality expert Brian Little in his 2014 book Me, Myself and Us.)

The Eysencks wanted to test Hans Eysenck’s own “cortical arousal” theory of extraversion and introversion. He proposed that this aspect of personality has a physiological basis and that introverts have higher baseline cortical arousal, which makes them react more strongly to stimulation; essentially, they feel things more intensely, perhaps causing them to shy away from certain situations. The Eysencks claimed that the lemon test supported the theory because people who score higher on questionnaire measures of introversion tend to salivate more in response to lemon juice.

Introverts tend to respond more strongly to loud noises

Although the personality dimension of introversion/extraversion is certainly influenced by biological factors (including being partly inherited from our parents), we now know that the arousal theory itself is only half true. There is ample evidence, including from brain imaging studies, to show that introverts tend to respond more strongly to loud noises and other sensory stimulation. But contrary to Eysenck’s theory, there is precious little evidence that introverts have higher baseline levels of arousal in general.

So while it’s still debatable whether the lemon test can accurately reveal your introversion, it certainly does tell you something interesting about your physical sensitivity – and you could always try repeating it a few times to get a more reliable result.

In any case, extraversion and introversion aren’t the only aspects of personality that can be measured with a lemon-based test. A paper published in 2014 suggested that we can also use a lemon to test a different aspect of personality – how much empathy people have. Again, this is something that psychologists often measure using questionnaires, with the usual issues of subjectivity and honesty coming into play.

To provide a more objective test, Florence Hagenmuller and her colleagues asked volunteers to put three rolls of cotton in their mouths (used to measure saliva) and watch two one-minute videos – one featured a man cutting up and eating a lemon, the other (the control condition) involved the same man taking coloured balls out of a container and putting them on a table.

Afterwards, the researchers weighed the cotton rolls and found that, overall, the participants salivated more when watching the man eat lemon than the control video. This is an example of what psychologists call “autonomic resonance” – the way that we automatically mimic each other’s physiological states, such as when we yawn when we see someone else yawn, or wince at their pain. But we all differ in how sensitive we are in this respect and, intriguingly, the researchers found that the higher the participants had scored on a questionnaire measure of empathy (they agreed with statements like “I often have tender, concerned feelings for people less fortunate than me” and “I am often quite touched by things that I see happen)”, the more they tended to salivate while watching the man eat lemon.

This is a trickier test to try at home, unless you want to start weighing cotton rolls filled with saliva! Also, to get any meaning from the results, you’d have to compare how much you salivated to the lemon video compared with one or more other people. I could imagine it would make for a fun science experiment to try out at school or college.

The “lemon test” could assess empathy in people with schizophrenia or autism

Hagenmuller and her team said that the lemon-based empathy test could have some serious uses – for example, to assess empathy in people who can sometimes find it difficult to follow questionnaire instructions, such as people with schizophrenia or autism. The advantage of the lemon video test is that it doesn’t require any comprehension on the part of the test-taker. All they have to do is sit back and watch the video, and the results provide a read-out of how much empathy they experience at an automatic visceral level.

So next time you find yourself in a possession of a bag of lemons, you’ve got a choice. You could make lemonade or you could start doing some psychology experiments. I wonder what your decision says about your personality?

--

Dr Christian Jarrett edits the British Psychological Society's Research Digest blog. His latest book is Great Myths of the Brain.

Join 500,000+ Future fans by liking us on Facebook, or follow us on Twitter, Google+, LinkedIn and Instagram.

If you liked this story, sign up for the weekly bbc.com features newsletter, called “If You Only Read 6 Things This Week”. A handpicked selection of stories from BBC Future, Earth, Culture, Capital, Travel and Autos, delivered to your inbox every Friday.