According to this study in Pediatrics, children are more likely to choose food with a 'Shrek' or other character on the label than ones without. This is something food marketers were well aware of. Also, interesting in the study is that the 'Shrek effect' is lower for healthy foods than unhealthy ones. That is all very well but then, the study's authors making a flying leap into policy:
Overall, our results provide evidence that licensed characters can influence children’s eating habits negatively by increasing positive taste perceptions and preferences for junk foods. Given that 13% of marketing expenditures ($208 million) targeting youths are spent on character licensing and other forms of cross-promotion, our findings suggest that the use of licensed characters on junk food packaging should be restricted.
In fact, the study shows no such thing. It shows that labeling can induce a preference for less healthy foods but it says nothing about whether that will lead to more consumption of those foods. That depends on price and also on parental-child relative bargaining powers which can easily mean that a Shrek label allows parents to negotiate a lower consumption of unhealthy foods.
To be sure, it may be that the Shrek effect implies all of the problems the paper conjectures but that has to be studied directly. For instance, is it the case that when offered a deal between 2 carrots and one Shrek labeled 25g chocolate versus 1 carrot and plain labeled 50g chocolate, the children always the latter? What precisely is the rate of exchange? We need to know that before jumping to policy conclusions.
At the moment, the study does explain why unhealthy foods may attract labels while healthy ones do not. So it is a useful contribution to the business of food marketing.