I chose the title for this piece for a number of reasons that will become apparent below. It's no surprise that because I've been thinking about positive emotions and feelings for a long time, often focusing on play behavior, I began thinking about different reasons why joy has evolved—what's it good for—rather than if it has evolved. Some hardline skeptics still aren't sure, but this simply means they're not keeping up with comparative scientific research on joy and having fun, a topic to which an entire issue of the journal Current Biology was devoted. Another important up-to-date source is an excellent essay published a few days ago, titled "Animals experience joy. Scientists want to measure it" by Amber Dance.

Thinking about joy as a unifying principle brings together a lot of data and ideas from different—sometimes surprising and unexpected—areas of study. This is clear for nonhumans and humans, something I learned from an interview I did with Lybi Ma about her recent book, How to Be Less Miserable: End the Negative Mind Loops and Find Joy.

What is joy? Here, I'm using a working definition of joy offered by my long-time colleague, Dr. Colin Allen. Dance writes:

"To keep it simple, Allen and his colleagues have focused on a strict definition of joy as an intense, brief, positive emotion triggered by some event, such as encountering a favorite food or a reunion with a friend. That kind of 'woohoo!' moment seemed easier to assess than, say, ongoing mild contentment. Even with a strict definition, the researchers are contending with variations in joy triggers and responses from one animal to the next, including within the same species or group."

There are some common elements linking joy across different and diverse species, and Dance offers a good number of examples, including parrots making snowballs while making playful “warble calls” that are contagious, like human giggle fits, that trigger joy, dolphins leaping through water, blowing bubble rings, and playing catch with seaweed.

The idea of viewing joy as a unifying principle bringing together somewhat of a potpourri of ideas from studies of play and other activities seems like a very reasonable idea. What caught my attention in the title of Dance's piece, and why I chose my title for this essay, centers on the idea and challenge of measuring joy. Let's consider a few areas of research in which ideas about joy and how and why it evolved can be used. Building on Dance's examples, I'll say a few words about play behavior.

Numerous diverse animals play, and it surely seems........

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