Give a person the right resources and, within an hour or so, they could dramatically change their physical appearance. They might dye their hair or shave it all off. Change the colour of their eyes. Wear a prosthetic nose. Fake teeth. Seven-inch heels. The list of potential superficial transformations is endless.

But if we go beneath the skin into our psychic architecture, is it possible to change the joists, supporting walls and foundations or who we are? What we believe, value and aspire to in life? How we behave, patterns we form? How and why our hearts break?

On many levels, it’s easy to just say “yes” to this question. The stages of cognitive development a child goes through before the age of 11 reads like a mini lifetime. Our entire emotional landscape changes when all our hormonal switches are flicked on and we hit puberty. For women who menstruate or those approaching the menopause, hormonal fluctuations can cause changes in mood and resilience levels throughout our cycles. People’s sexual preferences can change over time. Where someone identifies on the spectrum of gender might change. A first-time parent’s worldview might change after their baby is born.

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We know that psychological trauma can have profound, lasting effects, not just on our state of mind, but even the way our cells behave. We also know that the brain is plastic and changes throughout our lives. There is a body of evidence that tells us regular mindfulness meditation exercises can lead to measurable changes in brain areas and networks associated with positive effects such as decreased anxiety. When we experience distress and ask for help, we know that interventions such as psychological therapy and medication can, in time, bring about a significant shift in how we’re able to cope with the gullies and obstacles of life.

When a wave of depression breaks, we can reconnect with who we know ourselves to be. No mental state is concrete. The mind is always in flight. There would be no such thing as psychological therapy in the first place were there not centuries of study founded on the belief that because our brains are so clever, we human beings have the fundamental ability to analyse our thoughts, feelings and behaviours in order to evolve. Adapt. Learn what has caused us pain and fear and, hopefully, suffer less and live life more fully.

Facebook Twitter Pinterest ‘We know that the electrical, chemical brain is not extricable from the subjective mind.’ Photograph: Alamy Stock Photo

There is a lot to be read these days about cutting “toxic” people, who will never change, out of your life to bring you a sense of peace. If we apply this way of thinking to abuse within relationships, a clean break can, in some cases, be a matter of life and death. Whether or not a person continues to be an abuser hopefully becomes a criminal justice matter. Within the system, you hope that rehabilitation is possible and further trauma is prevented. In this sense, you pray that people can change.

We know that the electrical, chemical brain is not extricable from the subjective mind. Grey and white matter are constantly processing information and making decisions about emotional responses. This is where the concept of personality comes in. Our individual differences in patterns of feeling, thinking and behaving are significant because personality is often what we talk about when we think someone has changed.

Despite being encased in bone, these Daedalian structures we carry around inside our heads are quite fragile. To injure the brain, whether that injury is traumatic or acquired, can leave someone with significant changes in their emotional reactions. This may be perceived by those around the injured person as a personality change – something I’ve thought about a lot recently since I began working at Headway, the brain injury association.

At Headway I’ve spoken to members who, alongside any impact on mobility or sensory processing, talk about feeling like a different person to who they were before their injury – an observation largely drawn from what others have said about their personality. To me, these conversations show just how much general perceptions of human behaviour depend on comparisons. But so does most of life. As Iris Murdoch said in her essay The Sovereignty of Good: “The development of consciousness in human beings is inseparably connected with the use of metaphor.”

If the essence of a person is their personality, can it really be understood in terms of befores and afters? Can it, in some cases, even become disordered? If so, who gets to decide that it has?

This very question is at the heart of the controversy and activism surrounding the DSM-5 diagnosis of “borderline personality disorder” – thought by many psychologists to be a diagnosis of invalidation and oppression, particularly towards women who, invariably, are trauma survivors. Although this “diagnosis” may feel like a helpful prism for understanding someone’s behaviour – I have heard people say this – if they have hurt us in some way, when that diagnosis is founded on very little evidence we must not take its “criteria” at face value.

More generally, whether or not our personalities can change is a quandary that goes to the very heart of human nature. Do they change throughout our lives or are they set in stone? As with most things psychological, there’s no neat answer. It’s still very much an active research question.

Personality is no foolproof predictor of behaviour, but it does give us some general ideas about how a person is likely to think or act. A major meta-study published this year provided further evidence that personality can change with treatment. The research showed that four of the “big five” personality traits showed statistically significant change through life. This is in contradiction to William James’ famous assertion that, after 30, personality is set like plaster. However, participants’ reported personality “change” after treatment may be explained by returning to how they functioned before the problem they sought help with in the first place began to feel unmanageable. Studies like this also raise questions about how – and why – we seek to measure who people are in the first place.

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I have the suspicion that, if I am typing this question into a search engine, I am most likely seeking some kind of clarity or explanation for behaviour that has caused me pain. Of course, there are no definitive answers to be found. Certain theories or platitudes – “A leopard never changes its spots” / “Once a cheat, always a cheat” – might stick for a while, but if we decide that someone is fixed in their ways are we not doing so because, on some level, creating an immovable barrier between their behaviour and our pain is a kind of protection against more pain? If we decide that our own behaviour is unchangeable, is it not because saying “this is just who I am” is easier than opening ourselves to questioning? To taking responsibility?

Everyone has the ability to behave selfishly, callously and without care. We mess up and cause unbelievable pain to one another. But while it can be oddly soothing sometimes to decide someone is bad, the more we subscribe to the notion that a person can literally be or become a contagion, the more we move away from the wildly complex reality of human relationships and human character. We all bring our own tapestry of experience to the table and, sometimes, relationships don’t work out. The fabric stops fitting and there is rarely a precise reason for it. If we are seeking some kind of change in how we exist in relation to other people in this world the real question, annoyingly, is how much examination of ourselves we are prepared to do.

• Eleanor Morgan is author of Anxiety for Beginners: A Personal Investigation, and is currently training to be a psychologist