The question a Harvard psychologist says you need to ask yourself if you're feeling unfulfilled

On an episode of The James Altucher Show, Harvard psychologist Susan David shared a somewhat disturbing observation: We're affected by other people's behavior in ways we don't even realize.

On a superficial level, that might mean we're more likely to order candy on a plane if the person sitting next to us does so.

But a more insidious form of this "social contagion," as psychologists call it — and the one that really interests David — is that we can end up living someone else's dreams, instead of our own.

Here's what David, who is the author of "Emotional Agility," told Altucher:

"We can often land up being in a way in our lives where we turn around and we say, 'How did I get here?' I was just going on with the flow. I was just doing what everyone else told me to do."

In David's example, you go to college, get a job, and buy a house — i.e. hit all the traditional markers of success — but you still don't feel happy and fulfilled in the way that you'd like.

The problem here may be less that you acknowledged what matters to you and actively ignored it, and more that you don't know what matters to you.

The first step to feeling better about your life is to find out.

At the end of the day, David said, ask yourself: "What did I do today that was worthwhile?" Or, alternatively, "If today was my last day on earth, what would I have done that was worthwhile?"

(David is quick to note that you shouldn't ask what brought you joy, because the two types of activities often don't overlap.)

Behind David's insights is a growing body of research on the power of values-affirmation. About a year ago, I reported on research cited in Harvard psychologist Amy Cuddy's book "Presence," which found that writing about what matters most to you — family, creativity, whatever — helps you perform better in a challenging situation.

Specifically, participants in one study who completed the values-affirmation exercise were asked to choose from a list of values the ones that were most important to them and write about them.

On the podcast episode, David highlighted a few studies that show the power of values-affirmation to boost success in the long term as well.

For example, first-generation college students who did the values-affirmation exercise received better grades than their peers who didn't do the exercise. And female surgical residents who did the exercise performed better on clinical evaluations.

In both cases, the values-affirmation exercise seemed to prevent the activation of societal biases that could have inhibited the participants' success.

When you name the things that are important to you, David said, you are "more likely to cultivate habits that are congruent with who you want to be in the world." In other words, your abstract values are translated into concrete everyday behaviors.

In a way, the values-affirmation exercise is similar to writing a personal mission statement, a less-scientific idea that Stephen Covey put forth in his 1989 bestseller "The 7 Habits of Highly Effective People."

Some experts say that a personal mission statement is more effective than a New Year's resolution at helping you achieve your goals because you can connect to the "why" behind those goals.

Ultimately, the idea behind the personal mission statement, the values-affirmation exercise, and the question about what you did that was worthwhile, is to articulate values that might seem too broad or vague to be put into words.

"Values are often seen as being this abstract, arbitrary thing that people have on walls in businesses," David told Altucher. "But values are qualities of living. Values free us."