Although middleborn college students commonly report worse family relationships than other birth orders, it is unknown whether this effect persists into adulthood. Therefore, we investigated the effect of birth order on self-reported family and friend relationships among a large sample of Dutch adults (n = 794). Middleborns did not differ from other birth orders in relationships with their father, mother, sibling or close friend. Middleborns do not prefer a friend over their father, mother or sibling more than other birth orders. Evidence for a “neglected middleborn effect” appeared only in a within-family design for siblings. Firstborns were more likely to report very good sibling relationships and preferred a sibling over a friend. The results are discussed in terms of kin competition and inclusive fitness.