Our personality is supposed to be stable. Researchers say that enduring traits known as “The Big Five” best describe our personalities across cultures: extroversion, agreeableness (that’s compassion, respectfulness and trust); conscientiousness (organisation and responsibility); neuroticism (anxiety, depression and volatility) and openness to experience (imagination and curiosity).
Once we reach adulthood, scientists believed these five factors cohere into a largely permanent “I” with only modest changes thereafter. New personality research is revising those assumptions.
Birth order isn’t a serious driver of our enduring personality, says Ted Schwaba, a psychologist and assistant professor at Michigan State University who studies personality development and behavioural genetics. Rather than being fixed in the sky at birth, two decades of research suggests personality is far less static than once believed. “People always retain the capacity to change,” says Schwaba.
Our personalities shift – sometimes dramatically – throughout our lifetime in response to personal experiences, deliberate effort or the gentle sanding of years. The majority of people, says Schwaba, see a steady decline in neuroticism, our tendency to experience anxiety, sadness, worry, anger and guilt. “It’s the strongest evidence for any trait change,” says Schwaba, who confesses to anxiety himself, “which is the most reassuring finding in the whole world.”
Here’s how scientists believe birth order – and everything else – affects how we act and what really matters when it comes to our personality.
Missing link
Siblings promote more co-operative behaviour. That’s the middle child hypothesis: being sandwiched between brothers and sisters forges “peacemakers”, agreeable and honest personalities honed by navigating and soothing the needs of brothers and sisters.
To test this hypothesis, psychologists Michael Ashton and Kibeom Lee, professors at Brock University in Ontario and the University of Calgary, sifted through data from over 710,000 people to evaluate how birth order and family structure shape adult personality.
Instead of measuring the Big Five, Ashton and Lee used a slightly different measuring stick known as the HEXACO Personality Inventory. It essentially adds one trait to a modified version of the Big Five: Honesty-Humility, which translates into fairness, modesty and a reluctance to exploit others.
The results, published in the Proceedings of the National Academy of Sciences in 2024, appear to match what many might predict: those with both older and younger siblings score higher on honesty-humility as well as agreeableness. Those scores increased as the number of siblings in the household grew. Middle children scored highest, followed by younger siblings; firstborns and only children brought up the disagreeable rear.
But there was a catch: the overall effect was modest. Family size, the number of siblings a person has, mattered more than birth order, their position among those siblings. Even this only accounted for about 1% or less of the observed variation in honesty-humility and agreeableness scores in adults.
If birth order was shaping personality, the difference between siblings should show up as we grow older. That wasn’t happening.
So the authors of another study raised a second possibility: what if “personalities” we attribute to birth order are really just behaviours we adopt as children but then abandon after leaving home?
Thrill seekers
To test that idea, researchers studied whether younger siblings were more likely to engage in risky behaviour than older siblings. Their rationale was evolution: as siblings vied for parental resources and attention, they theorised, younger ones took risks to compete with stronger, more established siblings. The youngest’s evolutionary niche, you might say, was as the family wild child.
Tomás Lejarraga at the Max Planck Institute for Human Development in Berlin, and his co-authors analysed survey results from tens of thousands of children and adults between ages 10 and 39 in the United States and Germany and controlled for factors such as family size, sex, birth year and maternal education to see how consistently birth order predicted appetite for risk.
They uncovered the expected pattern: younger siblings between 10 and 14 years old were more likely to report risk-taking than firstborn children, according to the research published in the peer-reviewed Annals of the New York Academy of Sciences in 2023. But something strange happened. As respondents grew older, the signal faded. While birth order still (barely) affected someone’s appetite for risk in their 20s, the effect had vanished by their 30s. What this suggests is that the youngest’s reputation for recklessness is not a permanent personality trait, but a behavioural response to their environment. We change with it.
That’s in line with other studies over the past two decades that show our personalities are stable over years, but not fixed over decades. We’re much more interesting: we remain recognisably ourselves, especially over short periods, but our personality can change profoundly over a lifetime – especially if you decide you want to.
What actually changes personality
When I pressed Schwaba on when our personalities might finally mature during a lifetime, he demurred. “People are really not ever fully baked,” he said.
During our early years of life, personalities remain quite fluid. Witness high-schoolers. The “crucible” of personality development, say researchers, occurs between the ages of 18 and 25, after which stability steadily increases, plateauing at around age 30. Things then remain relatively consistent as adults engineer themselves stable careers, home lives and social circles that reinforce their personality.
After age 60, shifts in personality accelerate again, sometimes reversing gains in conscientiousness and extroversion. Yet something else extraordinary begins to happen at the same time. Emotional stability, the inverse of neuroticism, keeps increasing, and continues in our 70s, at least, even as our physical or social resources decline.
That process takes the fortunate into what researchers call the “paradox of ageing”: We achieve greater emotional meaning and wellbeing even as our physical conditions deteriorate. Challenges that might have once provoked negativity can instead be met with grace. “It looks like it takes us a whole lifetime to figure it out,” said Schwaba, “but we generally get less and less neurotic as we get older.”
What drives all this remains somewhat of a mystery. We do know some factors.
No one gene governs personality. But you can’t effectively explain personality without genetics. Twin studies suggest that 40 to 60% of the variance in personality traits between people, according to a landmark 2015 paper, is explained by our genetic code. Adopted siblings do not exhibit the same degree of personality similarities as those who are genetically related. “In other words,” said Schwaba, when it comes to personality, “you do see that siblings tend to resemble one another, but it’s not because they were raised in the same household.”
Good luck teasing nature and nurture apart, however. While genetics may deeply influence who we are, Elliot Tucker-Drob, a psychology professor at the University of Texas at Austin, warned that pinpointing the relative importance of environmental and genetic factors is not feasible. The two are so enmeshed that they likely magnify or suppress each other based on interactions we don’t (and perhaps can’t) fully understand – like trying to unmix a cocktail that’s already been made.
“Separating nature from nurture seems to be something that gets debated on internet forums,” said Schwaba. “It’s not actually the way that scientists are talking about it.”
Life events are another driver of personality. While not everyone experiences them in the same way, major life events have long been assumed to trigger shifts in the trajectory of several personality traits: divorce, childbirth, retirement, getting engaged, getting married and a demotion at work – in that order – show the largest association. That’s according to research that studied 25 different life events published by University of Missouri psychologist and professor Keely Dugan and her co-authors in the Journal of Personality in 2024.
But minor life events can change your personality too. Doing something special five times over three years for a spouse, for example, appears to have a similar (or greater) effect on personality as a major, one-time event such as getting married. “When minor events occur repeatedly over time,” writes Dugan,” their cumulative impact reaches a similar magnitude.”
It’s possible, she added, that “pride in one’s accomplishments and visiting with friends and family adds up to more than a wedding day or finalising a divorce”.
Finally, there’s sweat and tears. People do change but it takes dedicated time, effort and commitment. “There is good evidence that interventions can and do produce meaningful changes in personality,” said Tucker-Drob of the University of Texas at Austin, citing a meta-analysis of more than 200 studies of interventions from therapy or behavioural exercises, some of which led to “marked changes in personality” over time.
Reading self-help manuals is not enough. Regular, difficult work – a gantlet of behavioural challenges, such as concrete ‘if-then’ action plans, daily reflection journals, smartphone prompts and other actions – delivered meaningful and durable results, especially for neuroticism and extroversion, the study found.
“Where people decide to try to change themselves,” in those ways, said Schwaba, “they’re remarkably successful”.
You can’t trade in your personality wholesale. Broad patterns of thoughts, feelings and behaviours persist. While a timid person is unlikely to become the most gregarious in their social circle, most people will change over time. Despite the irascible exceptions, it’s usually for the better. “On average,” writes Rodica Damian, associate professor of psychology at the University of Houston, “everyone becomes more conscientious, more emotionally stable, and more agreeable”.
Less about who you are is fixed than we once thought. Our personalities are like landscapes: recognisable across the years, but steadily reshaped by the forces around us and, if we choose, by our own determined effort.