Bridging barriers: a mixed-methods study on using health communication to increase knowledge on mental health services among migrant parents in Sweden.

While mate choice (associated with a romantic partner) and parent-offspring relationships have obvious direct fitness relevance, the fitness benefits of friendships are more indirect. Friendships evolved principally to buffer us against the stresses of living in large social groups, and through that influence our health, longevity and fertility (). Having friends (as opposed to being a social isolate) has a dramatic impact on longevity, the ability to resist and recover from illness and injury, and fertility (see , ). This is true for both humans and other social mammals, including primates and equids: social females recover faster from injury, have lower cortisol titres (indicating lower stress levels), live longer, have higher social rank, are more fertile and have higher offspring survival rates (; ; , ; ), Unlike more conventional adaptive traits such as stature or competitive and problem-solving abilities, however, friendships depend on a reciprocated relationship to provide their benefits. As a result, friendships require considerable time investment, both to establish and to maintain (; ). How well these traits gel between individuals (the so-called homophily effect: ; , ) influences both the stability and the functional effectiveness of friendships. In this respect, friendships depend on psychological traits that often differ from those that make individuals attractive as mating partners (). This raises questions about the processes that underlie friendship formation and the role these play in the evolutionary process. Conventionally, evolutionary biologists differentiate explanations into four complementary levels – function, mechanism, ontogeny and phylogeny (Tinbergen's “Four Why's”: ). This is a reminder that it is important to consider mechanisms (the processes that enable fitness functions) when evaluating a trait's adaptiveness. The mechanisms that produce a function are an important part of the story. Here, we focus on the mechanisms that underpin friendship formation. Of particular importance in this context is the fact that the number of relationships we can maintain at any one time are strictly limited (). Hence, when moving into new social environments that offer new friendship opportunities, we are obliged to make decisions about whether to maintain old friendships or abandon them in favour of new ones. In studies of a major life transition (moving away from home to start college), we typically abandon old “home” friends (but not family relationships) in favour of new friendships, while retaining broadly the same number of relationships (; ). How we do this and what traits influence how well we do this then become of central interest. proposed that the “need to belong” (the desire to establish and maintain supportive social relationships, akin to ‘love and belongingness’) is one of our most fundamental evolutionary drives. Being socially well embedded has been shown to have very significant effects on both psychological physical health and wellbeing (; , ) in ways that will inevitably influence fitness. Feeling socially excluded triggers strong, immediate negative reactions that can have longterm negative effects (; ; ). For this need to be met, interactions have to be recurrent and generally pleasant. Furthermore, they must occur in a “temporally stable and enduring framework of affective concern for each other's welfare” (, p. 497).