Tests of behavioral inhibition at 14 months old can predict aspects of adult personality – the youngest age this has been shown.

“Give me the child until he is seven, and I will give you the man,” the old Jesuit saying goes. Many people have wondered if adult personality is set considerably earlier in life. Yet science has provided few answers, thwarted by the challenges of running studies that take decades to produce results.

Nevertheless, Dr Alva Tang of the University of Maryland and colleagues have tackled the question, assessing 165 infants at 14 months old and then again decades later. Inevitably, there was a substantial drop-off in participation, but the team write in Proceedings of the National Academy of Sciences that they "provide the strongest and earliest evidence showing that infants with an inhibited temperament at 14 months became introverted adults.”

Psychologists use behavioral inhibition (BI) to describe infants who, in the paper's words, show “overly cautious, fearful, and avoidant responses to unfamiliar people, objects, and situations compared to non-inhibited infants.” The same characteristics have been identified in the young of many non-human species.

Infants in the study with high BI were more likely to be reserved and introverted adults with fewer romantic relationships and they describe lower social functioning with friends and family. High infant BI was also associated with anxiety and depression in adulthood.

The findings could provide an opportunity for early interventions that reduce the risk of mental illness, although there are dangers in marking a child as bound for depression from such an early age.

Moreover, the authors acknowledge the size of the effects they found were modest. Many children with high BI were not reserved at all as adults, let alone prone to depression. Infant BI appears to be just one factor among many that shape adult personality.

Other scientists have shown BI tends to persist into early childhood, marked by greater social withdrawal and difficulties in forming peer relationships. However, even those studies started at age 3 or 4 and relied on parental surveys.

Tang not only started earlier, but had trained observers assess the infants' behavior in a new environment at 14 months, followed by conducting tests at 15. Participants' brainwaves were then measured with EEGs and surveys at 26.

The paper acknowledges the subjects of the study were not representative, being drawn from mostly upper-middle class suburbs in the Washington DC area and overwhelmingly white. Given their backgrounds, it is unsurprising that by 26, they were vastly more likely than the general population to have a university degree and be in professional employment. Infant BI had no significant effect on educational outcomes, career success, or even being in a romantic relationship when the survey was done.