Personality trait or mental disorder? The same genes may contribute to both

You don’t need fancy genome-sequencing or brain-imaging equipment to know that some of the people we know and love are just a little, well, out there.

We used to call these people “worriers,” “creative types,” “eccentrics” or “loners.” Like the rest of us, they seem to have come into the world with some recognizably fixed personality settings: They’re friendly or moody or dreamy or disorganized. They’re just more extremely so.

Increasingly, we’ve come to acknowledge that some people who occupy the outer fringes of those character types have psychiatric disorders that stand in the way of their living the lives they want. At some point, the pronounced traits they’ve had from the start got nudged over an invisible line by age, adversity or life’s escalating demands.

They have a mental illness: depression, attention deficit disorder, bipolar depression, schizophrenia.

This notion—that mental illnesses are largely inborn personality traits that get pushed into extreme territory by life experience—has just gotten some high-tech confirmation from researchers at UC San Diego.

Aggregating the genetic profiles of close to 261,000 people, a UCSD team led by neuroscientist Chi-Hua Chen has identified six regions of the human genome that are significantly linked to specific personality traits. And when they compared the genetic regions linked to certain personality traits with the genetic sites that can be linked to certain psychiatric disorders, they found some remarkable points of overlap.

Take, for instance, “neuroticism,” one of the “big five” personality traits that psychologists use to measure and describe the fixed behavioral tendencies that collectively make up our personalities. Someone who scores high in neuroticism is generally given to negative emotions such as sadness or anger or dread, whereas someone who scores low on this trait has a generally sunny disposition.

Psychologists have known for many years that high levels of neuroticism predispose a person to depression and anxiety. But Chen and her team found that, in the huge population for which they had detailed genetic information, the places on the genome where variations predicted neuroticism were the same places where variations appeared that were linked to major depressive disorder and generalized anxiety disorder.

The UCSD team also found a link between extraversion—a tendency to be talkative, friendly and highly social—and attention deficit and hyperactivity disorder (ADHD). Several of the sites on the genome that Chen’s team found to be associated with extraversion were the same sites that genetic studies have linked to ADHD.

Chen’s team also found “openness,” another of the five traits that psychologists use to define our personalities, to be closely linked to schizophrenia and bipolar disorder. In these large groups of people studied, the researchers found that those who are intellectually curious, highly creative, risk-taking and generally open to new experiences tend to have genetic variations in predictable places on the genome. But they also found that people with schizophrenia or bipolar disorder tend to have genetic variations in those same places on the genome.

To hunt for these patterns, called “genome-wide associations,” the UCSD team combed through data from four large troves of genetic data. Those came from the commercial genomics service, 23andMe; from the Europe-based Genetics of Personality Consortium; from British-based Biobank and from Decode Genetics, an Iceland-based human genetics company.