Original Article | Published:

Behavior and Psychology

Personal and social norms for food portion sizes in lean and obese adults

International Journal of Obesity volume 39, pages 13191324 (2015) | Download Citation



Portion size is an important component of dietary advice for weight control, but little is known about what portion sizes people consider ‘normal’. This study determined the effect of body mass index (BMI), gender, dietary restraint and liking of the food on personal and social portion size norms for a range of foods and the degree of certainty over the norms.


Thirty lean (BMI 20–25 kg m2) and 30 obese (BMI 30–35 kg m2) men and women (aged 18–60 years) viewed 17 different portion sizes of 12 foods on a computer screen on two occasions a week apart. Participants responded ‘more’ or ‘less’ to each photograph reflecting personal portion size preference or perceived portion sizes of others. Personal and social norms for portion sizes of each food were determined using the method of constant stimuli giving a sigmoidal curve of the probability of answering ‘less’ over a range of portion sizes. The slope of the sigmoid at the norm gave a measure of certainty about the norm. Regression models were used to examine the effect of BMI, gender, dietary restraint and liking of the food on personal norms, social norms, the relationship between norms, and the slopes.


Personal norms were significantly larger in the obese (P=0.026), men (P<0.001), those with lower dietary restraint (P<0.001), and those with higher liking for the food (P<0.001). Social norms were larger for women (P=0.012). The slopes at the norms were 30% shallower in the obese and in men (P<0.001).


Larger personal norms for portion size among the obese, men, those with lower dietary restraint and those with higher liking for a food imply greater consumption, which may undermine weight control. Shallower slopes for norms in the obese and in men may imply less clearly defined habitual portion sizes.

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SEF, HBL, ALA and SAJ designed and set up the study. HBL, KV, ER and SH conducted the research. HBL analysed data, interpreted results and drafted the manuscript. SEF, ALA, KV, ER, SH and SAJ interpreted results and critically revised the manuscript. We thank all of the volunteers who took part in the study. This study was supported by a program grant from the UK Medical Research Council (U105960389). This study was supported by a programme grant from the UK Medical Research Council (U105960389). The funders had no role in study design, data collection and analysis, decision to publish or preparation of the manuscript.

Author information


  1. Medical Research Council Human Nutrition Research, Elsie Widdowson Laboratory, Cambridge, UK

    • H B Lewis
    • , A L Ahern
    •  & S A Jebb
  2. Behaviour and Health Research Unit, University of Cambridge, Cambridge, UK

    • S E Forwood
  3. School of Psychology, University of Birmingham, Birmingham, UK

    • K Verlaers
    •  & S Higgs
  4. Psychological Sciences, University of Liverpool, Liverpool, UK

    • E Robinson


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The authors declare no conflict of interest.

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Correspondence to H B Lewis.

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