PhD defence of Iris Versluis on Thursday 21 April 2016

On Thursday 21 April 2016 Iris Versluis will defend her PhD thesis entitled 'Prevention of the Portion Size Effect'. Supervisor is Professor Philip Hans Franses (Erasmus School of Economics) and the co-supervisor is Dr. Esther Papies (Universiteit Utrecht). Other members of the Doctoral Committee are Professor Kirsten Rohde (Erasmus School of Economics), Professor Kort van Ittersum (University of Groningen) and Dr. Nicole Mead (Rotterdam School of Management - Erasmus University Rotterdam).

About Iris Versluis

Iris Versluis was born on June 12, 1984 in Sliedrecht, The Netherlands. She received her B.Sc. degree in Economics and Business (cum-laude) from Erasmus School of Economics. She received her M.Sc. degree in Marketing Research (cum-laude) from the same school. After her studies, Iris worked as a professional market researcher at agency SKIM for five years. Here, she specialized in discrete choice modelling and worked with various multinationals including Unilever and Procter & Gamble.

In 2012 she started as a PhD-candidate at the Erasmus Research Institute of Management (ERIM) under supervision of Prof. Philip Hans Franses. In 2013, Dr. Esther Papies joined the supervisory team. Iris’ main research interests are in the area of eating behaviour, focusing specifically on portion sizes, restrained eating, goal activation, health claims, and social norms. Her work has been published in Appetite and she presented her research findings on various conferences including the Annual Meeting of the British Food and Drink Group, the General Meeting of the European Association of Social Psychology, and the INFORMS Marketing Science Conference. Her work has also been covered in both national and international media, including Business News Radio, Reformatorisch Dagblad and Men’s Health.

Abstract of 'Prevention of the Portion Size Effect'

An increase in the portion size leads to an increase in energy intake, a phenomenon which is also referred to as the portion size effect. The increase in portion sizes in recent years is regarded as an important contributor to the increase in the prevalence of obesity. Hence, the aim of this thesis is to better understand why the portion size effect occurs and what can be done to prevent it.

We found that one of the reasons why people rely on the portion size when determining how much to eat, is because it is an indicator of what others will find an appropriate consumption quantity. To prevent the portion size effect, we explored whether it would be effective to remind diet-concerned people of their dieting goal. Such a reminder motivates diet-concerned people to control consumption, which makes it less likely that they will use the portion size as an indicator of how much they can eat. We indeed found that this reminder prevented the portion size effect. Finally, we showed that provision of a pictorial serving size recommendation can also weaken the portion size effect. A serving size recommendation provides people with an alternative reference point to rely on when determining their consumption amount, and hence they don’t need to rely on the portion size.

We conclude that with sufficient help, people will be able to rely less on environmental cues, such as the portion size, when making consumption decisions.

More information

Versluis' PhD research project is conducted within the Erasmus Doctoral Programme organized by Erasmus Research Institute of Management (ERIM), the joint research institute of Rotterdam School of Management (RSM) and Erasmus School of Economics (ESE) of the Erasmus University Rotterdam (EUR).

For more information about this ceremony, please contact Ronald de Groot, Communication Officer of the Erasmus School of Economics phone +31 10 408 1762 or by e-mail: