Look at a photo of yourself as a teenager and, mistaken fashion choices aside, it’s likely you see traces of the same person with the same personality quirks as you are today. But whether or not you truly are the same person over a lifetime—and what that notion of personhood even means—is the subject of ongoing philosophical and psychology debate.

The longest personality study of all time, published in Psychology and Aging and recently highlighted by the British Psychological Society, suggests that over the course of a lifetime, just as your physical appearance changes and your cells are constantly replaced, your personality is also transformed beyond recognition.

The study begins with data from a 1950 survey of 1,208 14-year-olds in Scotland. Teachers were asked to use six questionnaires to rate the teenagers on six personality traits: self-confidence, perseverance, stability of moods, conscientiousness, originality, and desire to learn. Together, the results from these questionnaires were amalgamated into a rating for one trait, which was defined as “dependability.” More than six decades later, researchers tracked down 635 of the participants, and 174 agreed to repeat testing.

This time, aged 77 years old, the participants rated themselves on the six personality traits, and also nominated a close friend or relative to do the same. Overall, there was not much overlap from the questionnaires taken 63 years earlier. “Correlations suggested no significant stability of any of the 6 characteristics or their underlying factor, dependability, over the 63-year interval,” wrote the researchers. “We hypothesized that we would find evidence of personality stability over an even longer period of 63 years, but our correlations did not support this hypothesis,” they later added.

The findings were a surprise to researchers because previous personality studies, over shorter periods of time, seemed to show consistency. Studies over several decades, focusing on participants from childhood to middle age, or from middle age to older age, showed stable personality traits. But the most recent study, covering the longest period, suggests that personality stability is disrupted over time. “The longer the interval between two assessments of personality, the weaker the relationship between the two tends to be,” the researchers write. “Our results suggest that, when the interval is increased to as much as 63 years, there is hardly any relationship at all.”

Perhaps those who had impulsive character flaws as a teenager would be grateful that certain personality traits might even out later in life. But it’s disconcerting to think that your entire personality is transformed.

“Personality refers to an individual’s characteristic patterns of thought, emotion, and behavior, together with the psychological mechanisms—hidden or not—behind those patterns,” note the authors, quoting psychology professor David Funder’s definition.

If your patterns of thought, emotions, and behavior so drastically alter over the decades, can you truly be considered the same person in old age as you were as a teenager? This question ties in with broader theories about the nature of the self. For example, there is growing neuroscience research that supports the ancient Buddhist belief that our notion of a stable “self” is nothing more than an illusion.

Perhaps this won’t surprise you if you’ve had the experience of running into a very old friend from school, and found a completely different person from the child you remembered. This research suggests that, as the decades go by, your own younger self could be similarly unrecognizable.