The psychology of food intake and portion control

October 2014

Numerous research has demonstrated the many and interacting factors impacting portion size and food intake. Some eating does occur in response to biological signals, such as hunger, but the majority of eating occurs more in response to psychological factors such as cognitions, hedonics, emotions, and our interpretation of our surroundings. This consumption in response to psychological factors tends to result in increased food intake and increased portions sizes.

Keywords: food intake, portion sizes, healthy eating, cognitions, hedonics, emotions, consumption norms, effort, distraction

This Information Statement was written by Dr Katherine Appleton, Bournemouth University


Overweight, obesity, and various associated poor health conditions continue to increase across the world [e.g. 1-3]. These increasing conditions have multi-factorial causes. One of the factors is increased food intake (e.g. 4). In developed countries, food is more readily available, food is cheaper, food is more accessible and easier to come by, than ever before [e.g. 5,6], but what factors determine what foods and how much of them people eat?

Reasons for consumption are many and varied. Some reasons for consumption are biologically based [7]. Consumption that stems from hunger and thirst is a direct response to a physiological requirement for fuel and/or fluids [7]. The majority of eating, however, is not biologically driven [8,9]. Much eating is a direct result of thoughts/cognitions, hedonics and emotions – reasons such as ‘I must eat now while I have the opportunity’, ‘I’ll have a second helping, because that cake is just so tasty’, and ‘I’ll have something nice for lunch to cheer me up’. Considerable eating is also a direct response to the environment in which we live - ‘I’ll have two of those, if the second one’s free’, ‘I always have a pudding on Sunday’, and ‘it would be a shame to waste it, now that you’ve made it!’. These reasons related to cognitions, hedonics, emotions and our environment can be classified as psychological reasons for food intake.

Eating in response to cognitions

Cognitions of relevance to eating include awareness of a healthy diet, of the reasons for consuming a healthy diet, or of government guidelines regarding healthy eating and health; knowledge of healthy foods, healthy diets, healthy portion sizes; and attitudes towards diet, health and the links between the two. Various observational evidence suggests positive associations between cognitions and healthy eating, such that greater awareness, greater dietary and health knowledge and more positive attitudes towards health and healthy diets are associated with more healthy diets and more healthy portion control [e.g. 10-14]. Similar effects are also found if awareness and knowledge are deliberately increased, or if attitudes become more positive [e.g. 15-18].

Eating in response to cognitions is often referred to as cognitively controlled eating, and varied evidence suggests that cognitively (or consciously) controlling food intake and portion size can be very effective. Every day examples of the successful cognitive control of eating  include vegetarians, vegans and individuals who restrict their intake of certain foods for religious reasons, and many individuals are able to successfully restrict their food intake to result in weight loss and successful weight maintenance [e.g. 19,20], some even to extreme degrees [e.g. 21]. Cognitive control, however, while potentially beneficial for dieting, health and a healthy intake, can also easily be overridden or disrupted, and this disruption can be problematic.

The first and now most widely recognized demonstration of this possible downside of cognitively controlling eating was by Herman & Mack [22]. In their study, 45 female students who classified themselves as either highly cognitively controlled eaters, or low cognitively controlled eaters  consumed either none, one or two milkshake preloads, and then took part in an ice-cream taste test. Importantly, individuals had no choice over whether they consumed none, one or two milkshakes, and they were required to consume their milkshakes in full. In the ice-cream taste test, individuals were free to consume as little or as much ice-cream as they wished, and amount consumed was measured. Results of the study are given in Figure 1. Firstly, individuals in the no preload condition demonstrate clearly the effective cognitive control of eating. Individuals who are highly cognitively controlling their food intake consume less ice cream than those who are not. The interesting results however are those from the two preload conditions. In these conditions, individuals who are not cognitively controlling their food intake consume less ice-cream after consuming a milkshake preload compared to those who had no preload. Individuals in the preload conditions have adjusted their ice-cream intake down to account for the earlier milkshake intake. The highly cognitively controlled eaters in the preload conditions however, not only fail to adjust their intake down, but end up eating more than those in the no preload condition. This effect is considered to result from disinhibition – the highly cognitively controlled individuals are considered to usually inhibit their desired / natural eating, and then this inhibition is destroyed by the ‘forced’ consumption of fattening milkshakes, resulting in what is often described as a ‘what the hell’ effect, where a dieter who has already broken their diet, might as well carry on and eat as much as they like [22].

This disinhibition has been demonstrated to occur in a range of individuals in a range of situations [e.g. 23]. Effects however are often larger, the greater the degree of cognitive control usually exerted, if the preload is perceived to be particularly fattening or particularly tasty, and if the individual at the time is particularly dysphoric or euphoric [22,23].


Figure 1: Amount (g) consumed in the ice-cream taste test (Herman and Mack (1975) [22, p. 656])

Number of Preloads                        0         1        2

Low cognitive control                     205     130    108

High cognitive control                    97       161    166


Eating in response to hedonics

The hedonic characteristics of foods refer usually to the tastes, flavours, and other sensory properties of foods that result in some foods being particularly liked, preferred or rewarding, while others remain less so. These hedonic characteristics typically result in increased consumption. Deliberate manipulations of tastes and flavours to increase liking have resulted in increased consumption [24,25], and increased hunger in general [24]. Greater familiarity with flavours and foods [26,27], and a greater variety of flavours and foods typically also results in increased consumption [28,29].

Eating in response to emotions

Like hedonics, emotions also typically result in increased consumption. Both naturalistic (diary-based observational) studies and deliberate manipulations demonstrate increased eating, increased portion sizes and often increased consumption of less healthy foods in response to positive emotions [30,31], negative emotions [30,32,33] and stress [34,35]. While increased eating is the norm, however, emotions can also result in decreased eating and eating more healthy foods in some individuals [23].

Eating in response to the environment

While cognitions, hedonic and emotions are obvious possible psychological determinants of eating, some eating occurs also as a direct response to the environment in which we live. One of the most interesting almost hidden determinants of eating and portion size is that of consumption norms - our understanding of the amount of consumption considered to be ‘normal’ for the situation in which we find ourselves. It’s normal, for example, to eat at certain times of day [36], it’s normal to eat certain foods at certain meals, and to eat certain portions of certain meals [37]. As a nice illustration of this, Kramer et al [37] provided individuals with breakfast food items and lunch food items for consumption at both breakfast and lunch. Breakfast foods were more liked than lunch foods at breakfast time and than at lunch time, and lunch foods were more liked at lunch time than breakfast foods and than at breakfast time. Individuals also consumed more of the lunch foods, regardless of whether they were consuming them at breakfast or lunch time, and consumed more at lunch regardless of whether they were consuming breakfast or lunch foods. These effects can be explained as a result of normal consumption patterns - breakfast is normally a small meal, composed of foods which we normally consume in small portions.

Portion size is particularly interesting - it’s normal also to finish your plate, to consume one portion - change the size of the portion, and you change the amount that someone consumes. Again as a nice illustration, individuals given 100%, 125% or 150% of a meal they previously self-selected typically consume all of that provided [38]. The effect is found furthermore if individuals are not only served a portion, but also if they serve the portion themselves. In a series of studies by Wansink and colleagues [e.g. 39,40], participants served themselves and ate 14.5% more ice-cream using a 3oz vs. a 2oz spoon, and 31% more from a 34oz vs. a 17oz bowl. Forty students at a Super Bowl party served and ate 53% more chips from 4l vs. 2l bowls, 161 moviegoers served and ate 53% more popcorn from 240gm vs. 120gm buckets, and 158 moviegoers served and ate 34% more stale popcorn from 240gm vs. 120gm buckets. Importantly here, participants were not given more food which they then consumed, they were simply given a larger popcorn bucket into which they served themselves and from which they subsequently consumed. These effects can again be explained as a result of normal consumption patterns where it is normal to take a full bucket of popcorn into the cinema, and then normal to eat it all, it is normal to take 2 or 3 spoonfuls of ice-cream to result in a bowl that is about half-full, regardless of the bucket, spoon or bowl size.

We also gauge how much we should be eating, not just from our micro-environment, but also from our macro-environment - by where we are, who we are with and how much they are eating. Independent of all other factors, individuals consume more in a restaurant than in a café, than in a cafeteria, than at home, than in a non-eating environment [41], individuals consume more with usual dining partners than with strangers or when alone [42], and individuals consume more when paired with a confederate who eats 40 crackers compared to a confederate who eats only 10 crackers [43] and consume more when paired with a confederate who eats 20 crackers, less when paired with a confederate who eats one cracker and an intermediate amount when alone [44].

Two other strong environmental determinants of eating are the effort required for consumption and the amount of distraction simultaneously provided. For effort, the more effort required for consumption, the less is likely to be consumed. In a cafeteria setting, where crisps were available at the check-out for the main cafeteria, crisps were selected on average by 70% of consumers. When crisps were placed at the far end of the dining hall and required queuing at a separate check-out solely for the purpose of purchasing crisps, crisps were selected by an average of only 10% of consumers [45]. In similar studies, consumers chose ice-cream less often (5% time) from a chiller with the lid on compared to a chiller with the lid off (16% time), consume less nuts if they are shelled vs. unshelled, and consume less sweets if they are wrapped vs. unwrapped [46]. Some of the effects above, as a result of portion size and serving size, are in fact not only effects of consumption norms but also effects of effort. Distraction furthermore, also explains some of the effects as a result of our macro-environment. We tend to consume more with familiar dining partners, not just because consumption here is normal, but also because these individuals are likely to provide greater distraction from the food we are eating – conversation is more likely, and the conversation is likely to be more engaging [42]. Distraction also explains an effect, where independent of all other factors, the more people we consume with, the more we consume [42]. Obvious effects of distraction are also available - individuals consume more in the presence of music compared to without [47], and with the TV on compared to off [48].


The determinants of eating, food intake and portion size are many and varied. None of these determinants also typically act in isolation, and usually it’s not just one factor that acts on each eating occasion, but many. The majority of psychological factors however, currently act to increase intake and portion size. While cognitive control can be effective for healthy consumption and healthy portion sizes, there is definitely an argument for a greater reliance on biological factors, and a greater awareness and use of biological signals.



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