Popular birth-order claims (firstborns are natural leaders, youngest children are rebels) have been tested extensively and largely don't hold up — large modern studies find birth order has little to no consistent effect on personality once family size and socioeconomic factors are controlled for. What does have real support is family systems theory's observation that families often assign functional roles regardless of birth order: 'the responsible one,' 'the peacemaker,' 'the scapegoat,' 'the golden child' — roles that emerge from family dynamics and stick because they solve a problem for the family system at the time, even at a cost to the individual playing that role.
The reason this matters in adulthood is that these roles are often unconsciously re-enacted in new contexts — workplaces, friend groups, romantic relationships — long after they stop serving any purpose. Someone who was 'the responsible one' as a child may find themselves automatically taking on disproportionate caretaking duties at work without ever having decided to, simply because the role feels familiar. Recognizing the family-of-origin role you were assigned is often the fastest way to notice when you're re-enacting it somewhere it's no longer useful — or wanted.
Rohrer and colleagues tested the birth-order personality claim against three large national datasets from Germany, the United States, and the United Kingdom, totaling more than 20,000 people — large enough, and with enough within-family comparisons, to settle a question smaller studies had argued over for decades. Testing birth order against the Big Five personality traits (extraversion, agreeableness, conscientiousness, neuroticism, openness), they found no meaningful effect on any of them once family size was accounted for. The one place they did find a small, real effect was self-reported intelligence — first-borns scored marginally higher — but nothing resembling the popular claims about firstborns being natural leaders or laterborns being rebels.