In the 1960s at Stanford University’s Bing Preschool, children were given the option of taking an immediate, smaller reward or receiving a delayed, larger reward by waiting until the experimenter returned. Since then, the "Marshmallow Test" has been used in numerous studies to assess delay-of-gratification. Yet, no prior study has compared the performance of children across the decades. Common wisdom suggests children today would wait less long, preferring immediate gratification. Study 1 confirmed this intuition in a survey of adults in the U.S. (N = 354; Median age = 34 years). To test the validity of this intuition, in Study 2 we analyzed the original data for average delay-of-gratification times (out of 10 min) of 840 typically developing U.S. children in three birth cohorts from similar middle-high socioeconomic backgrounds, tested 20 years apart (1960s, 1980s, and 2000s), matched on age (3-5 years) at the time of testing. In contrast to popular belief, results revealed a linear increase in delay over time, such that children in the 2000s waited on average 2 min longer than children in the 1960s, and 1 min longer than children in the 1980s. This pattern was robust with respect to age, sex, geography and sampling effects. We posit that increases in symbolic thought, technology, and public attention to self-regulation have contributed to this finding, but caution that more research in diverse populations is needed to examine the generality of the findings and to identify the causal factors underlying them.