This pairing has some good mojo behind it: The youngest child is cared for, while the older sibling can exert control. "The baby of the family tends to be the type who needs attention; the firstborn, who was alone for a while in the family, doesn't need to seek attention, because he or she usually got it, " says Dr. Salmon. Relationship Tip: Emphasize the relative strengths of your personalities. If you're married to a lastborn, don't disparage what you see as his lack of responsibility. Instead, go with him on some adventures. Conversely, if you're a lastborn married to an oldest child, you can learn how and why being serious can be a good idea.
Middle with Middle
Too bad Jan and Peter Brady couldn't marry! Their smack-in-the-center, sensitive, compromising natures would have given them an edge in keeping a relationship healthy. "In studies of marital satisfaction, middle children fare best all around, " says Dr. Salmon. Even so, if both of you tend to be the secretive type, you could have difficulty communicating. Relationship Tip: Have frequent, air-clearing conversations about everything from money and sex to the kids, home and work so your individual needs don't get drowned in a sea of compromise.
Youngest with Middle
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While as a rule, middles can usually have harmonious relationships with someone from any birth order, this combo may present some issues. That's because middles morph into the styles of the other types, depending on the dynamics of their particular family, says Dr. Salmon. A middle child with a much younger sib may act more like a lastborn (and the opposite situation may make the middle more like a firstborn). Relationship Tip: Try to suss out whether you have controlling tendencies (which you should keep in check so you don't overwhelm your younger-sib spouse) or if you both are acting like "babies."
Youngest with Youngest
These two can have a lot of fun—a pair of carefree, risk-taking lovers nearly always do. But the classic conundrum here is that no one wants to be in charge. "You may find that neither of you wants to handle the finances or make other important decisions, " says Dr. Salmon. Two last-born parents could be in a tough position: Both may prefer to be the kids' friend, not the heavy hand when it comes to discipline, which puts a strain on a marriage. Relationship Tip: Try to figure out which of you is best at certain tasks (such as handling money or making decisions about the children), and then own up to that responsibility, rather than assuming the other will take care of it.
Onlies with Anyone
Unlike the other birth-order positions, only children haven't been studied as much, says Dr. Salmon. "Most people assume an only child will resemble a firstborn in relationships, " since they are, after all, first, but that doesn't take into account the fact that an only never had an advisory (or bossy!) role with younger sibs. An only with a firstborn can be a good match if the only child acts less classically "firstborn." And an only with the lastborn can present issues, says Dr. Salmon, if the only has had little experience with the relatively immature, attention-seeking behavior of the baby of the family. Perhaps no surprise, middles and onlies make a good match, with the middle child accustomed to the needy side as well as the possibly bossy side, of his or her "only" love. Relationship Tip: If you're with an only, figuring out whether he's more like an autocratic first born, or a pampered lastborn, will help you work through relationship snafus more smoothly. And if you are an only, you may do well seeking out a partner of any birth order who has a clutch of siblings, if, says Cane, you were you were the type who always missed siblings in your own home.