Causal Pathways

A Shift in Societal Norms

Evidence has suggested that overweight is becoming the new ‘normal.’199

Although the weight status underestimation appears to be relatively stable across time, given that there is an increasing number of overweight individuals, the overall number of overweight adolescents who underestimate their weight status may actually be increasing.22 As the prevalence of overweight continues to increase, these trends emphasize the importance of societal characteristics (i.e. the average weight of individuals in one’s community) in influencing our own weight perceptions and consequently our weight. This concept can be explained using the Theory of Endogenous Weight Norms.200 This theory posits that people want to weigh less than the average individual and is based on two general concepts: (1) an individual’s preference to be thinner than the average person; and (2) that individuals tend to compare themselves to others. This theory can thus be summed up by recognizing that, as people become heavier, ideal weight also becomes heavier. That is, individuals prefer to be thinner than the average (relative weight), but the actual weight this preference corresponds to is increasing (absolute weight). As a result of these changing preferences, Burke and Heiland200 hypothesize that a normalization of a heavier ideal weight may be an important contributing factor in the current obesity epidemic. Perceptions of one’s weight are becoming increasingly based on subjective rather than objective criteria for overweight.199,200 This includes adolescents increasingly comparing themselves to their peers and less to external sources, such as the media.27 This theory is supported by evidence from a study conducted by Maximova et al195 finding that

adolescents exposed to overweight at school and at home (i.e. parental overweight) were at an increased risk of underestimating their weight status.

As an alternative to the Theory of Endogenous Weight Norms, Neighbors et al201 has suggested two competing hypotheses that may explain the adoption of overweight as a new normal: individuals are comparing themselves to those in their environment and those individuals are becoming increasingly overweight. Alternatively, women may be becoming more accepting of larger body sizes. These hypotheses are based on the Social Comparison Theory.

All of this has led to an increasing threshold for overweight.199 That is, the cut point or reference value individuals use to decide whether or not they are overweight is

increasing. It has been hypothesized that the current rates of childhood obesity may increase this threshold even further, resulting in further misclassifications of overweight status among those who are overweight.199 While the Theory of Endogenous Weight Norms and the hypotheses proposed by Neighbors et al201 are attractive explanations for this change, other possible explanations cannot be ruled out. These include the influence of the media and the effects of public health campaigns.199

Social Contagion of Obesity

It is evident that there is a relationship between exposure to overweight and risk of weight status underestimation, and that this relationship may play a role in the further propagation of overweight. This is supported by evidence suggesting that there is a contagious component to the obesity epidemic. Blanchflower et al202 found that there is evidence of a spread of obesity across European adults. Further, who someone compares his or her weight to depends on his or her sociodemographic characteristics. For

example, those most highly educated compare themselves to others who are also highly educated, a group of the population that tends to be thinner. Thus, individuals

demonstrate social comparison of their weight status to their peers. These comparisons are based on relative measures of weight status rather than absolute measures.

Trogdon et al169 found similar results among American adolescents. There was a high correlation between an individual’s weight and the mean weight of his or her peers, after controlling for demographic characteristics, smoking, birth weight, and parental and household characteristics. Adolescents whose parents have a high BMI are also more likely to be overweight. Trogdon and colleagues169 hypothesize three mechanisms that may lead to this contagious aspect of obesity. These are: (1) the direct effects of peers’ weight, ‘endogenous or causal effects;’ (2) characteristics of their peers other than their weight, ‘exogenous or contextual effects;’ and (3) factors common to both the adolescent and their peers, ‘correlated effects.’ Exogenous or contextual effects include

adolescent’s weight. Correlated effects are underlying factors that make an adolescent and his or her peers similar, such as a school exercise policy. Females and those with the highest BMI tend to be the most influenced by their peers, compared to males and those with a lower BMI, respectively. The authors hypothesize that increasing comparisons may similarly play an important role in the increasing prevalence of overweight among adolescents.169

Younger high school students are more likely to be overweight if there is a high prevalence of overweight among the senior students at their school.203 This effect

persisted when controlling for individual-level risk factors for overweight. However, this study did not consider school-level predictors of overweight. It has also been shown that the BMI of individuals in an adolescent’s peer group, particularly those of the same sex, are important predictors of increased risk of overweight.204 Using techniques to adjust for the bi-directionality of the relationship between peer and individual weight status, the effect of overweight status on individual weight remained significant for females only. This problem is exacerbated by the fact that overweight and obese adolescents are more likely to perceive that the average weight of their peers is higher than it actually is.205 Adolescents overestimating the weight of their peers are more likely to underestimate their own weight.205 As a result, these adolescents perceive the norm as being higher than it actually is, and may in fact see themselves as having a weight that is similar to that of their peers. These adolescents are consequently more likely to underestimate their weight status.