THERE'S NO SUCH THING AS ENVIRONMENT:

Scientists and parents have believed birth order shapes personality since the late 1800s. Psychological giants like Francis Galton, Alfred Adler and, more recently, Frank Sulloway suggested that firstborn children received special treatment and had greater power than their later-born siblings. As a mom of boys born one minute apart, I've often wondered whether the "firstborn effect" applied to twins.

"The idea that birth order affects personality has profoundly penetrated the parental consciousness," says Brent Roberts, a psychology professor at the University of Illinois at Urbana-Champaign. Parents across the globe say their oldest takes the lead, the middle child plays the role of mediator and the baby grabs attention at every opportunity.

But a spate of studies in recent years, including one co-authored by Roberts, has debunked the idea that birth order has any impact on personality. In 2015, a landmark study with more than 20,000 individuals from three different countries showed that where a child falls in the lineup makes no difference in terms of the Big 5 personality traits: extraversion, emotional stability, agreeableness, conscientiousness and imagination. Similarly, a 2019 study published in Personality and Individual Differences reported no evidence of a link between birth order and personality.

In an ironic twist, research suggests twins' personalities may develop, in part, based on who is larger and healthier at birth. As it happens, the firstborn twin usually snags those defining characteristics. [...]

Scientists have long viewed twin studies as the gold standard for exploring genetic and environmental influences on self-esteem and personality. But no household offers identical experiences and exposure for a set of twins. "The same family environment doesn't exist, even among twins raised in the same family," says Rodica Damian, assistant professor of social psychology at the University of Houston.

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