IN THE 4th century BC, the Greek philosopher Theophrastus, Aristotle’s student and successor, wrote a book about personality. The project was motivated by his interest in what he considered a very puzzling question: “Why it has come about that, albeit the whole of Greece lies in the same clime, and all Greeks have a like upbringing, we have not the same constitution of character?”

Not knowing how to get at the answer, Theophrastus decided to instead focus on categorising those seemingly mysterious differences in personality. The result was a book of descriptions of personality types to which he assigned names such as The Suspicious, The Fearful and The Proud. The book made such an impression that it was passed down through the ages, and is still available online today as The Characters of Theophrastus.

The two big questions about personality that so interested Theophrastus are the same ones we ask ourselves about the people we know: why do we have different personalities? And what is the best way to describe them?

In the past few decades, researchers have been gradually answering these questions, and in my new book, Making Sense of People: Decoding the mysteries of personality, I take a look at some of these answers.

When it comes to the origins of personality, we have learned a lot. We now know that personality traits are greatly influenced by the interactions between the set of gene variants that we happen to have been born with and the social environment we happen to grow up in. The gene variants that a person inherits favour certain behavioural tendencies, such as assertiveness or cautiousness, while their environmental circumstances influence the forms these innate behavioural tendencies take. The ongoing dialogue between the person’s genome and environment gradually establishes the enduring ways of thinking and feeling that are the building blocks of personality.

This developmental process unfolds, in part, through progressive changes in brain circuits, something largely completed during critical periods – windows of time that allow environmental and genetic processes to work together to build stable neuronal connections. For example, many aspects of language development take place during a window that remains open until around puberty. The difficulty in overcoming an accent in later life is a sign of the reduced brain plasticity that follows as this window is progressively closed.

On the other hand, certain programmed changes come online later in life, such as those that begin in adolescence and give rise to changes in personality and brain structure. Although it will be difficult to unravel the contributions that specific genes and events play in the decades-long process of constructing a brain and a personality, the general principles are now established.

We have also learned a lot about the second big question: how to describe personality differences. We now think of every personality as a unique blend of components. To use these discoveries to assess a specific person, it is useful to organise what we know about them into four sets of components: dispositional traits, troublesome patterns, character strengths and sense of identity. Bringing these together is, I find, the best way to build a descriptive picture of someone’s personality.

The first set of components, the dispositional traits, are ones that we intuitively pay attention to in sizing someone up – and they make a useful rough initial profile. For example, when we meet someone new we intuitively ask ourselves, are they assertive or reserved? Warm or cold? Organised or disorganised? Tense or relaxed? Open to new ideas or closed? Psychologists call these traits the Big Five, labelling them: extraversion/introversion; agreeableness/antagonism; conscientiousness/disinhibition; neuroticism/emotional stability; and openness to experience/closedness.

Once these prominent traits have been identified, the next step is to look for signs of potentially troublesome personality patterns psychiatrists have identified because they may bring grief to the individual or to the people he or she deals with. Amusingly, some of these patterns resemble those Theophrastus described. For example, his Suspicious type is similar to what psychiatrists call the paranoid personality pattern, in which other people’s motives are interpreted as malevolent. His Fearful type resembles the avoidant personality pattern, in which an individual is socially inhibited, with feelings of inadequacy and hypersensitivity to negative evaluation, while the Proud type resembles the narcissistic pattern, with someone exhibiting grandiosity, a need for admiration and low empathy.

As with dispositional traits, these patterns can be thought of in a graded, dimensional way rather than in the categorical yes-no mode that Theophrastus used. After all, you can be more or less paranoid, or avoidant, or narcissistic, or even some combination of all three. If you see signs of one or more of these patterns, take note. If you don’t, that, too, tells you something.

After noticing any potentially troublesome patterns, attention should shift from a simple description of dispositional traits to an assessment of their adaptive value – that is, is the trait or pattern helping the person get along in life or is it getting in their way? This also sets the stage for another type of judgement, using criteria that are explicitly moral. Unlike the adaptive criteria, which are based on observations of what works for a person, moral criteria are influenced, in part, by our instincts about good and bad.

Although cultures influence moral standards, every culture values self-control (temperance and courage), mutually beneficial relationships with other people (kindness and fairness), and a personal awareness of one’s place in the universe (self-transcendence and wisdom). The degree to which someone expresses these globally admired character strengths can, therefore, be taken as a measure of the “moral” structure of his or her personality, which is why I recommend adding this assessment to a personality profile.

The final stage in completing a description of a personality is to figure out the person’s sense of identity – their personal narrative of where they are headed and how they got to be the way they are. Grounded, in part, in reality, this personal story also includes plans for the future and selective reconstructions of the past. To understand a personality it is, therefore, necessary to observe this story as it is expressed in words and actions.

While the first three parts of the tool kit assessment outlined above provide a good description of someone’s behaviour, it is the person’s sense of identity that helps me grasp the meaning of their behaviour.

The steps I take to describe someone’s personality help me consciously think over the intuitive observations and assessments I make as I get to know the person. The result is a picture that incorporates much that researchers have found to be important.

Although this method oversimplifies, it is a practical way to prepare an explicit description of a personality. It should help us make sense of the people around us and may influence the approach we use to engage with them. I suspect Theophrastus would have approved. As a bonus, I find that this conscious descriptive process helps me more fully appreciate the complex humanity of those with whom I share my life.

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Samuel Barondes is a psychiatrist and neuroscientist based at the School of Medicine of the University of California, San Francisco. Among his books is Better than Prozac. This essay is based on his new book, Making Sense of People: Decoding the mysteries of personality (FT Press)