Although humans measure time using a continuous scale, certain numerical ages inspire greater self-reflection than others. Six studies show that adults undertake a search for existential meaning when they approach a new decade in age (e.g., at ages 29, 39, 49, etc.) or imagine entering a new epoch, which leads them to behave in ways that suggest an ongoing or failed search for meaning (e.g., by exercising more vigorously, seeking extramarital affairs, or choosing to end their lives).aging | meaning in life | time | decade | life-span development A lthough humans age continuously, many societies divide the human life span into 10-year periods, or decades. In the English-speaking world, for example, people describe these periods as "the twenties," "the thirties," "the forties," and so on, which implies that human aging progresses through discrete 10-year epochs. Because the imminent approach of a new decade signals the end of one era and the beginning of another, we examine how adults respond when they enter the final year of a chronological decade (i.e., when they reach the age of 29, 39, or 49 years, and so on, hereafter, "9-ending ages" and the people at those ages, "9-enders").We expect decades to play an outsized role in human psychology, just as discrete boundaries do in other domains. For example, round numbers occur more often than other numbers in literature (1) and function as behavioral goals (e.g., SAT takers are more likely to retake the SAT if they score just under rather than just over a round number) (2). We confirmed this intuition by asking 100 adults on Amazon's Mechanical Turk platform to indicate, in descending order, which 10 birthdays they considered to be the most significant throughout the human lifespan. Apart from 18 and 21, which are momentous because they signal the beginning of adulthood and the arrival of certain state-sanctioned rights and responsibilities, the most common responses were 30, 40, 60, 50, and 100. Moreover, the most common end-digit for all listed ages was 0 (46.4% of all responses), followed distantly by ages ending in 1 (14.8%), 5 (12.7%), 8 (9.6%), 6 (7.1%), 3 (5.2%), 2 (1.5%), 9 (1.1%), 7 (0.7%), and 4 (0.7%). In contrast to ages 18 and 21, 0-ending ages are momentous because they feel subjectively different from the ages that come before them, rather than because they impose objective changes on people's lives. Because the approach of a new decade represents a salient boundary between life stages and functions as a marker of progress through the life span, and because life transitions tend to prompt changes in evaluations of the self, people are more apt to evaluate their lives as a chronological decade ends than they are at other times (3-5). Consequently, although people may not routinely ponder whether their lives are meaningful, we believe they will be more likely to consider this question when they reach 9-ending ages.Once they do audit the meaningfulness of their lives, people tend to reach one of two conclusions: Either they conclude happily that their l...