Birth-order roles may play a part in how adults relate to each other in marriage. Two firstborns who marry will have to remember that both can not simultaneously insist on being the boss. They will have to temper their aggressive instincts with consideration and tolerance for each other, perhaps setting up in advance a system for problem solving of agreeing on areas of responsibility.
When two youngest children marry, both will have to remember that they cannot revert to the role of dependent baby, demanding everything be done for them. In fact, youngest children may have to remember, whatever the birth order of their spouses, that they cannot expect to be fussed over and pampered the way they were by older sibling.
Middle children have often learned the art of compromise to preserve family harmony. They must remember to speak up for their own desires and not to compromise themselves into unhappiness and dissatisfaction.