Because happiness is difficult to measure, modern studies expand on Aristotle’s work by focusing less on what happiness is and more on how happiness can be increased and sustained in our lives.

Sonja Lyubomirsky, PhD, a psychology professor at University of California–Riverside, conducts most of her research in a relatively new field called “positive psychology,” which she explains is about the positive side of life—what makes life worth living—rather than traditional psychology topics like depression, stress and how to fix things. She’s also the author of several groundbreaking books on the subject, including The How of Happiness (Penguin, 2008).

“Happiness is modern society’s holy grail,” Lyubomirsky says. “According to extensive research, most people around the world rate being happy as one of their most important life goals. I have always been struck by the capacity of some individuals to be remarkably happy even in the face of stress, trauma or adversity.”

Like Aristotle, Lyubomirsky believes that happiness is different for everyone, and her extensive research on what she terms “the architecture of sustainable happiness” has tried to answer the question: Why is that?

To understand the reasons, she suggests, think of happiness as being a whole pie; it’s a concept that was also explored in the 2011 documentary Happy. Approximately 50 percent of the pie is genetics; we are born with our own set point or base (like Aristotle’s golden mean) that we use when measuring how happy we feel we are. This ties into the Hedonic Treadmill theory, first written about in 1971 by psychologists Philip Brickman and Donald Campbell; it proposes that people always return to their level (set point) of happiness, regardless of what happens to them. Brickman later expanded on this theory by conducting a scientific study that compared groups of accident victims to groups of lottery winners and ultimately concluded that “happiness is relative.”

Through her research, Lyubomirsky has seen how a person’s set point comes into play when measuring happiness.

“My students and I have found that truly happy individuals construe life events and daily situations in ways that seem to maintain their happiness,” she says, “while unhappy individuals construe experiences in ways that seem to reinforce unhappiness.”

Remarkably, Lyubomirsky’s pie theory estimates that only 10 percent of our happiness is attributable to external circumstances like socioeconomic status, marital status, ethnicity, religious beliefs and age. This seems counterintuitive; people tend to assume circumstances play a much larger role in our happiness. But Lyubomirsky asserts there’s a wealth of evidence to support her conclusion, including a study showing that “Americans who earn more than $10 million per year are only slightly happier than nonwealthy office or blue-collar workers.”

That leaves 40 percent of the pie, and Lyubomirsky explains that this portion of happiness is completely under our control—our intentional daily activities and the choices we make. In other words, we can be happier if we actively work at it; there are happiness skills we can learn through practice.

“Happiness has numerous positive byproducts,” she says, “which appear to benefit not only individuals but families, communities and society as a whole.

Numerous studies have shown that people who are happy are more likely to succeed in life—in their work life, in their personal/family life and with their health. For example, happy people live longer, are more likely to find marriage partners and go on to earn higher incomes.”

How do you “get happy,” though, if you’re not? Lyubomirsky suggests taking that 40 percent of the pie—the large slice that’s completely in your own hands—and doing what she calls “Happiness Activities.” They’re all research-tested, “positive activity interventions” that you can repeat daily, for example, doing acts of kindness, nurturing your social relationships and learning to forgive those who have hurt you.

“Happiness strategies introduce you to the concept of mindful actions that you can use to achieve a happier life,” she says. “Apply them to your own life, and you will harness the promise of the 40 percent solution.”