It is often said that “birds of a feather flock together,” meaning that people tend to form personal relationships with others who are similar to them in important life aspects, which is also known more officially as the “similarity-attraction effect.” Although this effect has been researched for many years, few have examined the extent to which similarities of dietary habits can influence interpersonal attraction and the formation of relationships.

In this study, the focus was on vegetarianism, as it seems to serve as a social identity for individuals following this diet. According to previous research, vegetarians hold numerous different values compared to omnivores, including more pro-social attitudes and beliefs. These differences suggest that vegetarians should find other vegetarians more attractive as romantic partners and friends, especially as the similarity-attraction effect is stronger for attitudes than for personality characteristics.

The research was broken down into four parts, with four different sample groups:

1. A nationally representative sample of U.S. adults (n = 411) were asked about the salience of vegetarianism as a social identity.
2. A sample of U.S. undergraduate students (n = 721) described diets of their five closest non-romantic friends.
3. A sample of U.S. undergraduate students (n = 496) described diets of their three closest non-romantic friends.
4. A nationally representative sample of Poles (n = 863) described diets of their three closest non-romantic friends as well as of romantic partners.

For the first study, the scientists hypothesized that for vegetarians, diet is a more important part of their identity than for omnivores (study 1). In the second, third and fourth studies, they hypothesized that vegetarians would have more vegetarian friends and romantic partners than omnivores (studies 2,3, and 4). In all studies, vegan, lacto-vegetarian, lacto-ovo-vegetarian, and pesco-vegetarian diets were classified as vegetarian, and subsequently referred to as veg*ns throughout the paper.

Study one — using a 7-point Likert scale ranging from “1 = not at all important” to “7 = extremely important” — showed that diet was more important to veg*ns than it was for omnivores (average score of 5.53 vs 4.04). Furthermore, following a 7-point Likert scale ranging from “1 = almost never” to “7 = extremely frequently, a few times each day,” veg*ns thought about their diets more often than omnivores (average score of 5.63 vs 3.52). Here, the researcher conclude that dietary habits were more important for the social identity of veg*ns than for omnivores.

In studies 2 and 3, the researchers found their hypothesis confirmed that veg*ns were more likely to be friends with veg*ns than omnivores (30.6% vs 10.5%). Data from study 4 further reinforced this tendency, showing that within a Polish sample, veg*ns were six times more likely than omnivores to have friends who were also veg*ns (41.1% vs 6.4%). Even more participants stated that they were in a romantic relationship with other veg*ns, compared to omnivorous participants (52.5% vs 4.1%). 

Following these results, the researchers concluded that the similarity of a diet influences the formation or maintenance of relationships, which they attribute to the similarity-attraction effect. Since veg*ns experience diet as part of their social identities and the pro-social values play such a central role in vegetarianism, veg*ns may be more likely than omnivores to see diet as an important aspect of relationships. However, the researchers admitted that it may be problematic to lump all different subgroups of vegetarians together as dietary habits seem to be even more important for vegans than non-vegan vegetarians. 

Since the similarity of diet seems indeed to be important when making new friends, dietary habits may also restrict the formation of relationships. Here, the authors suggested that members of both participant groups may avoid personal relationships with each other, as former research indicated that their values cannot be reconciled. 

Since eating is often a group activity and plays a big role in people’s social lives, friends or colleagues may be excluded from communal meals because of the difficulty to satisfy everyone’s dietary preferences. However, to what extent people adapt their diets to the dietary habits of their relational partners needs to be examined in future studies.