Comments by Suzanne Higgs, Helen Ruddock, Lenny Vartanian and Jeff Brunstrom, September, 2019

In the 1980s and 1990s, while working at Georgia State University, John de Castro published a series of studies based on data from food diaries. de Castro found that people reported eating much more in the presence of others than when eating alone, a phenomenon known as ‘the social facilitation of eating.’ The size of this effect was far from trivial - meals eaten with others were reported to be 44% larger than those eaten alone (de Castro & de Castro, 1989). Although de Castro was not the first person to study how eating with others affects food intake, these studies served the field by demonstrating the powerful influence of social context, and this provided an impetus for a research area that continues to thrive today.

Prior to de Castro’s research, social influences on eating were studied in laboratory-based experiments. For example, Nisbett and Storms (1972) showed that people tend to model their intake on others: participants were observed eating 25% more food in the presence of a high-intake ‘model’ (a confederate of the experimenter who was instructed to eat a large amount) than when alone. Similarly, Polivy, Herman, Younger, and Esrkine (1979) reported that participants eating with a ‘high intake’ model consumed 31% more snacks than those exposed to a low-intake model. Building on this work, de Castro’s diary studies were critical because they showed that these observations also apply in the ‘real world.’ Indeed, they demonstrated that social facilitation is one of the most powerful determinants of energy intake in free-living humans (de Castro & de Castro, 1989).

Why does social facilitation of eating occur? The answer remains unclear, but a key observation is that facilitation is also observed in non-human animals, including rats (Harlow, 1932) and gerbils (Forkman, 1991). In fact, a study from the 1920s showed that eating behaviour could be reinstated in a satiated chicken if it was exposed to a hungry chicken that was eating (Bayer, 1929). Together, this literature demonstrates that social facilitation is unlikely to be driven solely by local social and cultural practices, and that observations in humans reflect a ‘lower level’ phenomenon that confers a broader biological advantage. de Castro went on to show that social facilitation of eating occurs at all meal types (breakfast, lunch and dinner) and eating occasions (at home or away from home and at both weekend and weekday meals) (de Castro 1991). These results were important because they demonstrated that the social facilitation of eating is not an artefact that arises because people eat more during specific social occasions, e.g., meals taken at the weekends or celebratory meals taken with alcohol (de Castro 1991). He also found that manipulating the number of companions affected intake: participants consumed significantly more calories over a five-day period when they were instructed to eat only with other people, compared with when they were instructed to eat all of their meals alone, or to eat as they normally would (Redd & de Castro, 1992). These findings suggest that eating in a group causes intake to be facilitated. Similar effects were subsequently observed in the laboratory: when participants were required to eat a test meal with others, they ate significantly more than when they ate alone (Clendenen et al. 1994). More generally, this body of work should be considered ‘gold standard,’ because it illustrates how stronger inferences can be drawn by using a range of methods that produce converging evidence.

In the paper presented here (de Castro, 1994), participants (515 adults) were asked to complete pocket-size diaries for seven days. They were asked to provide information about what was eaten at each eating occasion and the context in which the food was consumed, including who was present. A key finding is that meals eaten in the presence of others are larger and longer in duration than are those eaten alone (this effect was observed regardless of meal type and time of day), but that this social facilitation is more pronounced for meals eaten with family and friends than for meals eaten with co-workers or other dining partners (e.g., roommates and classmates). The study also found that social-facilitation effects on eating differed as a function of the participant’s sex, and the sex of their co-eater(s). Specifically, women eating with men showed a greater social facilitation of eating than did women eating with other women. In contrast, men showed similar social-facilitation effects on eating regardless of whether they ate with men or women. Finally, the study considered whether social-facilitation effects on eating could be explained by the effect of other people on participants’ mood. Participants endorsed higher ratings on a “depressed-elated” scale when eating with all types of companions, but particularly so when eating with co-workers and others. Eating with co-workers and others was also associated with greater anxiety. These data indicate that the social facilitation of eating is unlikely to be explained by an increased level of positive emotionality related to the presence of others (eating with co-workers had the biggest effects on “elation” (the top of the rating scale) and anxiety but the smallest effect on intake). Furthermore, because men ate more in a group regardless of whether they were eating with men or with women, modelling effects alone provide a poor explanation of the social facilitation of eating. Were this the case, then men should have eaten less when eating with women, because women tended to consume smaller meals. The finding that social-facilitation effects are more pronounced when people eat with friends and family was interpreted as a ‘disinhibition effect.’ de Castro speculated that participants may have felt more relaxed with familiar others, which reduced dietary restraint, leading to increased intake.

We recently conducted a systematic review and meta-analysis which confirmed that the social facilitation of eating is more likely to be observed when participants dine with familiar others than when they dine with strangers or acquaintances (Ruddock, Vartanian, Brunstrom and Higgs 2019). We also found that the effect of social facilitation on food intake (when eating with friends) (Cohen’s d=.76) is considerably larger than that of portion size (d=.45) (Zlatevska, Dubelaar, & Holden, 2014), and is similar to the large effect reported for modelling of eating (d=.85) (Vartanian, Spanos, Herman, Polivy, 2015). However, our review also revealed that the specific mechanisms underlying the social facilitation of intake have yet to be established with certainty. Another outstanding question is whether the social facilitation of eating is compensated for in the longer term. The importance of this issue should not be understated. In a single meal, the effect of social facilitation on food intake is large, but its effect on overall energy balance will be trivial. If social facilitation occurs often, however, and if larger meals go uncompensated, then the chronic impact on bodyweight could be substantial.

Of course, there are limitations to the use of food diaries, including issues with misreporting and under-reporting – problems that de Castro himself acknowledged (de Castro 2000). But, as de Castro also pointed out, there are strong arguments for studying ingestive behaviour in naturalistic settings. Questions around the most appropriate methods to study human eating have long been debated (e.g., Meiselman, 1992). Laboratory-based studies allow us to manipulate defined variables and to take precise measurements. But these alien environments (to participants) lack ecological validity and might introduce demand characteristics (Robinson et al., 2018). By contrast, naturalistic studies can provide data over longer durations and may be useful in assessing the relative impact of multiple variables in a range of real life contexts, but are limited by the challenges of measuring consumption in free-living individuals. de Castro and others (e.g. Rolls & Hetherington 1990) have advocated for the use of multiple approaches to the study of human eating and for the development of measures that provide a better indication of what people eat, day to day (de Castro 2000). Technological advances in our ability to unobtrusively and accurately track the eating behaviours of participants over days and months are likely to bring important new insights, and could play a vital role in exposing how and why social facilitation of eating occurs.

The efforts of John de Castro and others, notably Peter Herman, Janet Polivy, and Patty Pliner, have been critical in documenting the important role that social context plays in human eating. On this basis alone, their contribution to the field deserves our recognition. Their work also demonstrates the importance of drawing inferences from converging lines of evidence. Following their example, and with the advent of novel methods, the topic of social facilitation is likely to remain a fruitful and stimulating area of research for many years to come.

References

Bayer, E. (1929). Beiträge zur Zweikomponententheorie des Hungers. Z. Tierpsychol., 118, 283-349.

Clendenen, V. I., Herman, C. P., & Polivy, J. (1994). Social facilitation of eating among friends and strangers. Appetite, 23(1), 1-13.

de Castro, J. M. (1991). Social facilitation of the spontaneous meal size of humans occurs on both weekdays and weekends. Physiology & Behavior, 49(6), 1289-1291.

de Castro, J. M. (1994). Family and friends produce greater social facilitation of food intake than other companions. Physiology & Behavior, 56(3), 445-455.

de Castro, J. M. (2000). Eating behavior: lessons from the real world of humans. Nutrition, 16(10), 800-813.

de Castro, J. M., & de Castro, E. S. (1989). Spontaneous meal patterns of humans: influence of the presence of other people. The American journal of clinical nutrition, 50(2), 237-247.

Forkman, B. (1991). Social facilitation is shown by gerbils when presented with novel but not with familiar food. Animal behaviour, 42, 860-861.

Harlow, H. (1932). Social facilitation of feeding in the albino rat. The Pedagogical Seminary and Journal of Genetic Psychology, 41(1), 211-221.

Higgs, S., & Thomas, J. (2016). Social influences on eating. Current Opinion in Behavioral Sciences, 9, 1-6.

Meiselman, H. L. (1992). Methodology and theory in human eating research. Appetite, 19(1), 49-55.

Nisbett, R. E. & Storms, M. D. (1972). Cognitive and social determinants of food intake. In Thought and Feeling: Cognitive Alteration of Feeling States, pp. 190-208 [H. London and R. E. Nisbett, editors]. Chicago: Aldine Publishing Co.

Polivy, J., Herman, C. P., Younger, J. C., & Erskine, B. (1979). Effects of a model on eating behavior: The induction of a restrained eating style 1. Journal of Personality, 47(1), 100-117.

Redd, M., & de Castro, J. M. (1992). Social facilitation of eating: effects of social instruction on food intake. Physiology & Behavior, 52(4), 749-754.

Robinson, E., Bevelander, K. E., Field, M., & Jones, A. (2018). Methodological and reporting quality in laboratory studies of human eating behavior. Appetite, 130, 321-326.

Rolls, B. J., & Hetherington, M. (1990). A behavioural scientist’s perspective on the study of diet and behaviour. Diet and behaviour: Multidisciplinary approaches. London: Springer-Verlag, 209-217.

Ruddock, H. K., Brunstrom, J. M., Vartanian, L. R., & Higgs, S. (2019). A systematic review and meta-analysis of the social facilitation of eating. The American Journal of Clinical Nutrition.

Vartanian, L. R., Spanos, S., Herman, C. P., & Polivy, J. (2015). Modeling of food intake: a meta-analytic review. Social Influence, 10(3), 119-136.

Zlatevska, N., Dubelaar, C., & Holden, S. S. (2014). Sizing up the effect of portion size on consumption: a meta-analytic review. Journal of Marketing, 78(3), 140-154.