New study sheds light on the evolutionary origin of disgust toward older adults

According to a series of three studies, there is a stronger negative emotional and avoidance reaction toward unfamiliar – compared to familiar – older adults, confirming source effects of disgust toward this population. However, there were no moderating effects of filial piety – i.e., the virtue of respect towards elders. This research was published in Evolutionary Psychology.

Disgust is an important factor that links the behavioural immune system – “a defense system selected under the persistent disease pressures in an evolutionary environment” – with ageism. This system overgeneralizes at times, with people often perceiving abnormal body shape, movements, or behaviours as cueing the existence of pathogens. Aging is accompanied by physiological changes, including in appearance, posture, or movement (e.g., wrinkles, tremors). These changes could signal unhealthiness and prompt disgust toward older adults.

Some studies suggest disgust is stronger toward unfamiliar (vs. familiar) individuals, a phenomenon referred to as the “source effect of disgust.” Thus, it appears that the disgust experience can change, depending on the source of the disgust. In this work, Quan Cao and colleagues examined the source effect of disgust and avoidance responses toward older adults specifically. An important consideration is filial piety, a virtue in eastern cultures, which could be a moderating variable in the source effect of disgust.

Study 1 included 58 undergraduate students from a university in eastern China. Participants were randomly assigned to a “familiar” or “stranger” group. Participants in the familiar group were instructed to imagine and indicate (by typing) the older adult with whom they had the closest relationship (e.g., grandfather/grandmother) while those in the stranger group were instructed to imagine “a completely strange old man whose age is similar to that of your grandparents.” A total of 60 sentences were presented in random order, depicting disgusting scenarios (e.g., “someone emits a strong body odor”), and neutral scenarios (e.g., “Someone visits a friend who is ill in the hospital”). The subject of sentences were the titles that participants previously entered. For each sentence, participants rated their disgust feelings on an 11-point scale ranging from -5 to 5, with positive numbers indicating positive feelings. Afterwards, they provided demographic information.

A replication was conducted in a community sample of 82 older adults in eastern China. However, in Study 2, participants also completed the Filial Piety Scale, which is comprised of 12 items, such as “son and daughters should always show warmth to their parents”, “when the parents are sick, their offspring should arrange suitable treatment for the parents”, and “children should try their best to fulfill their parents’ expectations.”

Study 3 examined source effects on the tendency to avoid older adults with different levels of familiarity when they exhibit disgusting cues. The researchers write, “The behavioral immune system is a set of defense mechanisms for suppressing contact with living organisms that may carry the infectious pathogens and is mainly constituted by two mechanisms: detection of cues to the threats of pathogens and the reaction to the threat of the pathogen. Usually, this reaction is avoidance or exclusion to the source of the disease.” A total of 69 undergraduate students from a university in eastern China were included.

A shape discrimination task was used to measure the reaction time of participants’ approach / avoid actions. Participants were prompted to respond to shapes that appeared on the screen via keyboard as fast as they could. They were also presented with the same 60 disgusting/neutral sentences from the previous studies, authored by either a familiar or unfamiliar older adult. After each time a key was clicked, participants were to move their hand back to the original position.

Sentences were presented in random order, and after each sentence, a circle or triangle appeared. For circles, participants had to move their hand from the middle of the keyboard to the small keyboard to click one of 1-9 numeric keys; and for triangles, they were required to press either “q”, “w”, “e”, “a”, “s”, “d”, “z”, “x”, “c.” In this study, arm movement to the target on screen was the approach action, while arm movement away from the screen was the avoid action. A difference score in reaction time between the approach/avoid actions provided a metric of participants’ tendency to avoid. A higher score was indicative of slow approach action and stronger tendency to avoid.

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Across three studies, Cao and colleagues found evidence of a source effect on disgust toward older adults. They write, “When older people carry negative cues, familiar older people will induce a lower level of disgust than that of the unfamiliar older people and trigger a lower level of behavioral avoidance.” This makes evolutionary sense, given familiar people tend to come in contact more frequently, and are more likely to share similar microbial flora; higher familiarity also suggests greater immune similarity and reduced health risks. However, coming into contact with a strange older person would make one more vulnerable to unknown pathogens. Importantly, a source effect was found in both undergraduate and community samples, improving the ecological validity of the findings.

As well, filial piety did not moderate the source effect. “This result shows that though different individuals are varied in terms of filial piety, their disgust for the older adults is in line with the results of evolutionary mechanisms. That is, familiar older people cause lower levels of disgusted feelings than the strange older people.”

A limitation is that the study focuses on self-report and behavioural responses. However, disgust prompts changes in the nervous system, including decline in heart rate, and changes in the middle-insula and basal ganglia system; future research ought to examine such physiological markers. As well, this work focused on Chinese culture, and may not extend to individualistic societies (e.g., United States). Lastly, a control group of younger adults was not included; thus, the effect of age of target ought to be isolated in future studies.

The research, “Behavioral Responses to Familiar Versus Unfamiliar Older People as a Source of Disgust” was authored by Quan Cao, Jian Sun, Ming Peng, and Bin-Bin Chen.