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Are you always the same person?

MY MOM sometimes jokes that it is fortunate she didn鈥檛 meet my dad when he was in college, because she wouldn鈥檛 have liked him. She was (and is) a self-described goody two shoes. Dad not so much, but presumably even less so when keg parties were involved.

We know that we change over time. Our bodies grow, then age; we mature and our views shift; our memories sharpen and fade. Yet for most of us, our sense of self is seamless and continuous. You are the same old you, right?

Let鈥檚 start with the physical. Some of our cells, notably neurons in the brain, are with us from before birth, and can live more than 100 years. 鈥淢ost of the nerve cells in the brain are actually as old as we are,鈥� says molecular biologist at the Karolinska Institute in Sweden.

But most of our cells aren鈥檛. Some, including certain kinds of white blood cell, live for only days. How quickly our skin cells are replenished changes as we age, but in general it takes about a month. The notion that the liver regenerates every 40 days or so is a myth: our liver cells .

On the level of atoms and molecules, meanwhile, we are exchanging material with our environment with abandon. Think of your body like a grassy field, says Fris茅n. 鈥淚t鈥檚 the same lawn from year to year, but each strand of grass is completely different.鈥�

But what about less tangible aspects of you? This, after all, is where it matters to us. Losing a consistent sense of a 鈥渘arrative self鈥� is at best discombobulating, and at worst devastating when we observe it in ourselves or in our loved ones as a result of injury or neurodegenerative disease. Ultimately, our physical bodies and ever-eroding collection of memories are what we are made of. 鈥淚t鈥檚 all we鈥檝e got,鈥� says psychologist at the University of Essex, UK.

Always a new you

And yet even our long-lived neurons are constantly in flux, rewiring themselves to generate new thoughts, memories and states of mind. The simple fact is that what we learn, what we eat, how well we have slept and countless other things influence our choices and behaviours all the time. So in many ways, 鈥測ou are not the same person from one moment to the next鈥�, says Gillmeister.

The illusory nature of the continuous self was backed up in 2016 when researchers at the University of Edinburgh, UK, investigated changes in the behavioural habits that make up our personalities across a span of 63 years. Previous studies, looking over shorter periods, found only small changes, suggesting that we largely stay the same. But the longer view was startling: measured over six decades, . We turn into different people over time.

Sometimes, people go through major changes all at once 鈥� 鈥渟omething big happens that turns their lives upside down and very thoroughly shakes them up鈥�, says psychologist , a co-author on that paper. Yet for the most part, our personalities drift through 鈥渄ribbles of change, conscious and not, in specific behaviours over long periods of time鈥�, she says.

We are strangely skilled at shifting our notions of who we were or what we believed to maintain an illusion of a continuous self. For example, we to get our previous attitudes to more closely match our current ones, dismissing the idea that we once held strong political views, say, with which we now disagree. 鈥淵ou make yourself up in the past,鈥� says Gillmeister.

At some level, we are also aware of the disconnect. Studies have demonstrated that we think about our future selves in a very different way to how we think of ourselves in the moment 鈥� in our brains, it is as if future you is a completely different person.

That might be something to work against: research also shows that simply thinking about the ways you will be the same person in the years ahead can , for instance. Maybe that is what swung it for Dad.