But is it selfish of parents to have only one child? It's hard to deny that having only one child shortchanges the only child out of one of the most important relationships an individual can have in this lifetime: the relationship with one or more siblings. As a therapist who has sat through almost a hundred family therapy sessions in his career, I can tell you that kids often feel that their strongest ally and most trusted partner in the family system is their sibling or siblings — even if they sometimes fight and insist that they dislike each other.