What Is the Oldest Daughter, Youngest Son Relationship Theory?

We each have our own unique biology and past experiences that shape our perception and behavior across various aspects of our lives—including our romantic relationships. One aspect of our upbringing that affects who we gravitate toward and how we show up as a partner? Our birth order. According to birth order theory (which dates back to the 20th century), an individual’s birth order in their family—whether they’re the oldest, middle child, or youngest—affects their personality, and, by default, what they look for in a partner.

From this lens, your birth order is said to influence your relationships, particularly which birth order you’re most compatible with—otherwise known as birth order dating theory, a topic that’s been going viral on TikTok this year. Based on this trending idea, firstborn females and last-born males are the perfect romantic match because their personalities and needs balance one another out. 

While TikTok has millions of posts that anecdotally back this theory, is it psychologically sound? We interviewed an expert, licensed psychotherapist and social worker Kirstin Anderson, to find out.

The Definition of Birth Order Dating Theory

Although the birth order dating theory is taking over TikTok at the moment, the concept has actually been around for over a century. In the 1900s, Austrian psychologist Alfred Adler posited the idea that a person’s birth order affects their personality and behavior in relationships—both familial and non-familial. Based on Adler’s theory, where someone falls in their family can explain why they’re attracted to certain people and why certain relationships prevail.

What to Know About the Oldest Daughter, Youngest Son Relationship Theory

Following the birth order dating theory, the most compatible romantic match is the oldest daughter and youngest son. Within this context, Anderson explains that responsible firstborns and easygoing last-borns have congruous needs, which makes them suitable partners. “When these relationships go well, the oldest daughter brings organization, caretaking, and nurturing to the relationship, while the youngest son brings adaptability, playfulness, and spontaneity,” says Anderson.

Oldest daughters have a history of taking care of their younger siblings, whereas youngest sons are used to receiving attention from their older siblings—so they each might seek out a partner who allows them to maintain those roles. Plus, firstborns typically bring a sense of organization and structure to a relationship, and last-borns add a sense of adventure, which creates balance. “While they can then appreciate the stability and care that their partner provides, they also are able to contribute flexibility, spontaneity, and fun to the relationship,” Anderson says of youngest children.

Is There Any Validity to the Oldest Daughter, Youngest Son Relationship Theory?

Anecdotally speaking, TikTok seems to agree that the oldest daughter, youngest son relationship theory holds true. From a psychological lens, Anderson notes that there is some credibility to the theory, but birth order isn’t the first and foremost determining factor of attraction and relationship success. “While birth order is definitely one trait that affects compatibility, it’s only one trait of many and shouldn’t be one of the most important factors when considering a prospective partner,” Anderson says. There are so many other elements that shape an individual’s choice of partner and behavior in a relationship, from childhood experiences to genetics.

And just because birth order can be influential doesn’t mean it always is. “While many of the characteristics dictated by birth order theory can be spot on, not everyone will always identify with the personality traits that the theory would expect them to have,” Anderson adds. Each individual and family is different, so an oldest child in one family might not identify with the stereotypical traits of a firstborn, whether it’s due to certain circumstances or values, per Anderson.

Are There Any Other Compatible Birth Order Pairings?

Although the firstborn-last-born match is the most well-suited, Anderson says another birth order pairing that may work is two middle children. “As middle children are often known for being easygoing and adept at navigating conflict, this pair can often understand each other well and often be on the same page,” she explains.

Two oldest children and two youngest children are often the least compatible, though, according to Anderson. “While different birth orders often can balance one another out and allow each partner to bring something unique personality traits to the relationships, too much of any trait can be a bad thing,” she warns. Two oldest children might vie for control, constantly butting heads over their struggle for power. And with two youngest children, their relationship might lack organization or caretaking.