Birth order myths aren't just fascinating cocktail-party talk. (You are totally a middle child!) There are solid psychological reasons why many people fit the mold. Here's a breakdown of the major stereotypes—plus the five "disruptors" that throw everything off.

The firstborn

Stereotype: Natural leader, ambitious, responsible.

Why it's true: The eldest, for a while, has no competition for time (or books or baby banter) with Mom and Dad. "There's a benefit to all of that undiluted attention. A 2007 study in Norway showed that firstborns had two to three more IQ points than the next child," says Frank J. Sulloway, Ph.D., the author of Born to Rebel. Firstborns tend to be surrogate parents when other siblings arrive, hence their protective and responsible nature.

When it's not: Parents can set high expectations for a first (or only) child. "When he feels like he has disappointed his parents or can't measure up, he may veer off in another direction," says Kevin Leman, Ph.D., a psychologist and the author of The Birth Order Book.

The middle child

Stereotype: Social butterfly, peacekeeper, fairness-obsessed.

Why it's true: "Middle-borns don't have the rights of the oldest or the privileges of the youngest," says Catherine Salmon, Ph.D., a coauthor of The Secret Power of Middle Children. As a result, they become experts at negotiation and compromise. They also tend to lean on their friends, as their parents' attention is often focused on the oldest or youngest child.

When it's not: If the oldest doesn't act the part, "it creates a job vacancy," says Salmon. "Donald Trump is a middle with a firstborn brother who didn't fit the role. Donald usurped it." And what if there are several middle children? "There's a principle that each child is trying to be different from the one immediately older," says Salmon. "So if you had three middles, the first and third would likely be a bit more similar to each other than to the very middle child."

The baby

Stereotype: Free spirit, risk taker, charming.

Why it's true: Parents are less cautious. (Hey, the older ones ate the dog's food and lived!) And they also probably have more resources than they did when starting out. "Parents are more lenient, so youngest kids tend to be less rules-oriented, and yet they still get lots of attention," says Salmon.

When it's not: "Some babies resent not being taken seriously," says Linda Campbell, a professor of counseling and human development at the University of Georgia, in Athens. "They might become very responsible, like the oldest, or social, like the middle."

5 things that throw it all off

Don't feel like your birth order? You're not alone. According to the White-Campbell Psychological Birth Order Inventory (or PBOI)—a test developed to measure whether people are a "fit" for their rank—only 23 percent of women and 15 percent of men are a true match. Here's why.

1. Temperament

Nothing affects personality development more than genetics. Roughly half of your personality is the temperament you're born with, says Sulloway. And that's why where you fall in your family or how early you had to start sharing blocks is only part of the pie. A child's temperament can trump birth order—or at least blur the lines. Firstborns, in particular, are expected to succeed at whatever the family prizes most. (Son, you come from a long line of politicians...) So when they aren't well suited, it turns into a sibling free-for-all. "If the firstborn can't excel at what the family values, for example, that position could shift to another child," says Salmon.

2. Gender

"Gender is a significant influence when it comes to the birth role that one develops within the family," says Alan E. Stewart, Ph.D., who researches birth order at the University of Georgia. For example, Andrew is a by-the-numbers, achievement-driven firstborn. When Annie comes along, she doesn't have to worry about carving out her own identity or living in his shadow the same way that another boy would. She's already fundamentally different. The result: "When the first two children are different genders, they often both behave like firstborns," says Leman. In fact, the second-born could even eclipse the eldest. "If there's a high value being placed on one gender over the other, the dynamic gets disrupted," says Salmon. (P.S. In larger families with a lone girl or boy, that "exotic" status will also enable the child to escape his or her stratum.)

3. Physicality

Age and size advantage frequently go hand in hand, so older kids get to boss younger ones around because they're bigger. Except if you happen to have a slight eldest child or an especially robust middle or youngest. In that case, the power dynamic can flip-flop, says Leman.

4. Specialness

Yes, yes, they're all special. But when one child is a violin prodigy or an Olympic gymnast, she tends to get the prime treatment (and pressure) of a firstborn, no matter her actual spot. "For the chosen one, being special will negate other birth-order things, like middle-child syndrome," says Leman. If that happens, other siblings must adjust. Other circumstances can alter families, too. "A child with a disability who needs extra care can disrupt the sibling dynamic," says Campbell. A grandparent living in the home can also shift things—if one kid gets extra attention from Nana, say, or takes on a caretaker role.

5. Age spacing

"The closer the age intervals are between siblings, the more competition there is," says Stewart. When kids are one to two years apart, especially if they are the same gender, there's more conflict, says Salmon. (Good news: That doesn't mean they won't be close when they're older.) Parents are also overwhelmed, which adds to the turmoil. The closely born second child may overtake the firstborn role by being better, faster, and stronger—or zig to her zag. (She likes to dance? I'm going with softball.) Three to four years tends to be a sweet spot; kids are close in age but have room to be themselves, says Salmon. Many experts agree that five or more years between kids acts as a reset button, kicking off a "new family" with a fresh firstborn. And the former youngest, now middle, may never shed the baby role. "If you're a second child whose sibling is 10 years older, then in most practical ways you grew up as a firstborn or only child," says Sulloway. What about twins? Rules don't apply. "Twins are the special focus of their parents," says Salmon. "There's typically less competition between identical twins. Fraternal twins, however, behave more like other siblings."