– There's no doubt about it: The human body goes through major changes in youth and old age. And these biological and social changes may be the reasons behind what researchers are calling less stable personalities at those life stages. In fact, according to a study of almost 4,000 New Zealanders between the ages of 20 and 80, the stability of one's personality increases through youth and into middle age and peaks in our 50s, before we slide back into less stable versions of ourselves, reports Research Digest. As the Smithsonian puts it, "in some ways, our 80-year-old selves mirror our 20-year-old selves." Some traits peaked at different times; the most stable traits in one's 30s are extraversion and neuroticism, while in the late 40s and early 50s it is openness, honesty-humility, and conscientiousness. The only trait that showed "gradually reduced stability" throughout life was agreeableness; apparently we are simply less and less agreeable as we pack the years on. Still, one's most basic personality—introvert vs. extrovert, for example—tends to remain consistent throughout one's life, even if the stability of traits waxes and wanes, notes the Smithsonian. (Other researchers have even calculated the country's most extroverted city.)