I had always assumed that things like meaning, mission, and purpose were higher-level needs, but the more I learn, the clearer it becomes that Meaningful work is a basic human need that crosses occupations and income levels. SC: You write about the importance of positive social interactions. What does this mean for introverts, who often prefer to spend social time in smaller doses, with closer friends and punctuated by solitude? TR: There's nothing more appealing to this introvert than a combination of solitude and a little time with my closest friends. Perhaps this is why research on ratios of positive and negative interactions has always intrigued me. The quality of social interactions matters much more than the frequency of those interactions. From what I've studied, we need about 80% of our interactions with others to be more positive than negative. This is simply because negative interactions carry a much heavier load and outweigh positive interactions.