The idea that our personalities largely stop changing around the age of 30 is not new.

It was in 1890 that the theory was first brought to public attention with Harvard psychologist William James’ text The Principles of Psychology:

“In most of us, by the age of thirty, the character has set like plaster, and will never soften again,” he wrote.

But is this really true? Are we unable to change once we reach the ripe old age of 30?

According to psychologists, there is some truth in this notion.

“Personality by its definition is a relatively stable set of patterns of thoughts, feelings and behaviours, compared to moods, for example, which are much more temporary in nature, occupational psychologist Kirsten Godfrey told The Independent.

“It's understood that this reaches a peak of stability following adolescence and into early-mid twenties so yes, to some extent, by the time you're thirty your personality has fully formed.”

And she’s not alone in this opinion:

“The bottom line is that global personality traits tend to remain very stable over time, and certainly from age 30,” professor of psychology at the University of Texas, David Buss, explained to The Independent.

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Many people find their habits and personality traits harder to change in their 30s:

“I’ve spent a decade on self-improvement and the habits that I nailed down when I was in my early 20s are far more enduring and stable than the ones I’m working on now,” 31-year-old Richard* told The Independent.

“I certainly feel that as time passes you settle into routines which are much harder to change. Part of that is because life becomes busier as I get older, but also my brain seems more fixed.”

Occupational psychologist Carol Rothwell confirmed this: “It is true that as we age we find it more difficult to develop and some people become more stuck in their ways. Some people even stop developing as young as teenagers.”

Psychologists tend to break personality traits down into five categories: openness, extraversion, agreeableness, conscientiousness and neuroticism. These are our core traits which aren’t affected by moods, and various studies suggest they’re genetic.

When we’re maturing, however, these traits are still forming. By the age of 30, the majority of people have reached maturity.

But according to Buss, that doesn’t mean the five traits are completely set in stone. He says that after the age of 30, people generally become less neurotic (and thus more emotionally stable).

And many people over the age of 30 can attest to that idea of feeling more secure about themselves:

“I don't think it's that [personality] becomes more fixed, it just seems that way because over 30 you're more comfortable being yourself more often rather than changing to please other people,” 35-year-old Sarah* told The Independent.

“When you get over 30 you grow into your own skin. You start to know yourself far better,” added 38-year-old Lisa*.

And 37-year-old Stu* agrees: “I have a better understanding of who I am and now I am less willing to compromise my core beliefs.”

But as well as decreased neuroticism, Buss says there are two other personality changes which tend to occur after the age of 30:

“People generally score higher on agreeableness with increasing age (they get less aggressive or antagonistic),” he says, and similarly they tend to score slightly higher on conscientiousness - “they become somewhat more responsible or dependable.”

Rothwell believes it’s a complicated and complex issue:

“Who we are is not a product of a fixed blueprint or our age - rather it is a combination of experiences and our opportunity and motivation to develop our full potential,” she explains.

“Life experiences and who we meet through life influence our personality. Good experiences and nurturing people tend to accelerate the rate at which we can access our potential. However poor or bad experiences can also be character building.”

She points out that we’re more malleable when we’re younger so our experiences shape us more.

Godfrey agrees: “The way I think of personality is more like a unique hand-crafted instrument - every one is unique to each of us. It's shaped to some degree from the moment we are born and undergoes most of its fine-tuning throughout our early years through the experiences we encounter.

“Nevertheless, even past the age of 30, this instrument can continue to fine-tune throughout our whole lives and can be particularly affected by major life events, such as having children, starting a new job, or at the other end of the spectrum suffering a bereavement.”

So rather than being “plastered” by the age of 30, perhaps it’s more like our personalities are “half-plastered.”

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“It’s not that personality is fixed and can’t change,” said American psychologist Paul T. Costa Jr. “But it’s relatively stable and consistent. What you see at 35, 40 is what you’re going to see at 85, 90.”

And, Costa has said, there’s nothing especially magical about the age of 30, so you needn’t panic about hitting the big 3-0 and your personality being set in stone.

“But if you look at it from a developmental view, you can see the wisdom of [William James’s provocative statement],” he said.

A 2003 study actually claimed to completely disprove James’s “plaster theory”: “We found a general lack of support for the plaster hypothesis, and considerable evidence that directly contradicted it,” said Drs. Sanjay Srivastava and Oliver P. John of the University of California at Berkeley.

For most people, once you reach your 30s, life generally starts to settle down and become more constant. This means that for your personality to be significantly influenced, “it’ll take some relatively powerful change in the environment,” according to Costa.

You can, however, with some effort, work to change your personality later in life. It just might be harder than when you were younger.