Grimacing Or Smiling Can Make An Injection Feel Less Painful

By Emma Young

If you’re preparing to receive a flu vaccine — or even a COVID-19 vaccine — this winter, you’ll be interested in the results of a new study that investigates whether it’s better to smile or grimace your way through the pain of an injection.

The idea that manipulating our facial expressions can affect our emotions has a long and storied history. There are many advocates of this “facial feedback hypothesis”, and many critics, too. Indeed, one of the classic findings in the field — that people find cartoons funnier if they hold a pen between their teeth, inducing a smile — recently failed to replicate. This mixed research background was well known to Sarah D. Pressman and Amanda M. Acevedo at the University of California, Irvine, who led the new work, published in Emotion.

The team randomised 231 student participants to hold either a neutral expression, a regular smile (which involves the cheek muscles), a Duchenne smile (which also recruits muscles by the eyes, resulting in creasing around the eyes) or a grimace (which recruits both these sets of muscles, plus those which allow us to wrinkle our brows).

To induce these expressions, the participants (who thought they were taking part in an investigation of multi-tasking) were given chopsticks to hold in one of four positions between their teeth, and asked to imitate an example photo. Each participant then held their assigned facial position while they received an injection of saline solution into the upper arm, designed to mimic a flu jab. Their heart rates and skin conductance were measured throughout, as objective markers of their stress levels, and they also reported on levels of pain (anticipated and then actual) and other emotions, such as being jittery or relaxed.

The researchers found some interesting differences between the groups. The Duchenne smiling and grimacing groups gave lower pain ratings than the neutral group while waiting for the injection, during the injection, and immediately after it. The regular smiling group fell somewhere in between. Notably, the Duchenne group reported 40% less pain during the needle insertion than the neutral controls, and the grimace group reported 39% less pain. These were clinically meaningful results, the team stresses, equivalent to pain being “very much improved”. In addition, the Duchenne group had the lowest heart rates during the study, while the grimacing group reported feeling the lowest levels of stress.

Smiling and grimacing recruit some of the same key facial muscles, and both are seen in people undergoing negative experiences, including pain. The facial feedback hypothesis would hold that both these facial configurations act to regulate feelings of pain and stress. In the case of Duchenne smiling, it might be that we associate this expression with feeling positive emotions, and this reduces perceived pain. With grimacing, it’s possible, the team suggests, that intense activation of muscles around the eye and forehead has a more direct effect on pain and/or levels of physiological arousal, and stress, perhaps via stimulation of the trigeminal nerve in the face. But the nature of this mechanism, if it exists, is certainly not clear.

In relation to their grimacing findings, the team writes: “To our knowledge, this is the first experimental test showing that this natural response to pain is helpful in improving the subjective pain experience.” However, they also add: “Whether we grimace during pain because it is similar to smiling or whether we smile during stress because it is like a grimace warrants further investigation and replication.”

This is a controversial field of research. And there’s plenty more that needs to be done to understand the effects reported in this study. However, the pain-reducing effects of either Duchenne smiling or grimacing are dramatic, and as the team also notes: “With 10% of the [global] population estimated to be needle-phobic and 16 billion injections occurring every year, the benefits of this simple method could be far-reaching.”

Smile (or grimace) through the pain? The effects of experimentally manipulated facial expressions on needle-injection responses.

Emma Young (@EmmaELYoung) is a staff writer at BPS Research Digest