More than a third of all U.S. adults have a side hustle, recent data shows. The people who make the most money have something in common: lots of emotional intelligence.

That's according to Kyle M.K., author of the 2019 book "The Economics of Emotion" and a talent strategy advisor at job search website Indeed.

"A gigantic portion of [side hustle] work involves emotional intelligence pretty heavily," says M.K., who ran Austin-based customer experience consulting firm The Heart Company for eight years before joining Indeed.

"Every business is made up of people and every business serves a person," he continues. "So if we don't get really good at understanding people, understanding ourselves, it makes It a lot more difficult for us to do the work that we're hoping to do."

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Emotional intelligence — also referred to as emotional quotient, or EQ — is a useful skill for developing business strategies and customer-facing tasks, M.K. says.

The more you understand what other people need in their lives, the more you can build your product or service to be a useful solution, he notes. Thinking about your audience as real people — not data points — can help you learn where they like to shop, how much they're willing to pay and what else they might want to buy.

Personally connecting with customers can help your sales too, says M.K. To bring in consistent revenue over time, Twitch streamers need to connect with their viewers, Etsy and Amazon sellers need to provide great customer service, and Uber drivers need gauge the comfort levels of their passengers.

In other words, if you invest in creating a positive emotional experience for your customers, they'll come back for more, regardless of the size of the business. "I think [emotional intelligence] is going to be a big pillar of the future of work, especially with AI taking care of a lot of operational things," M.K. says.

How to practice your EQ skills

Developing your emotional intelligence may benefit more than just your side hustle: It can help you become more creative, form deeper relationships and lead to greater fulfillment, Stanford University lecturer and psychologist Emma Seppälä told CNBC Make It in April.

To better understand how other people process the world, try asking them emotion-led questions like "How are you feeling about..." or "What are your thoughts on..." during conversations, EQ researcher Jenny Woo wrote for CNBC Make It in July. Only ask about topics you genuinely want answers to, because you won't gain any empathy for other people if you don't listen to their responses.

Improving your self-awareness helps, too. You have to understand your strengths and weaknesses to succeed at work and maintain relationships with anyone, former Google vice president Claire Hughes Johnson wrote for CNBC Make It last year.

Hughes Johnson suggested a three-step strategy for becoming more self-aware: Write down your values and the ideas that give you energy, think about how those values contribute to your skills and deficiencies, and — if you get stuck — ask trustworthy peers or friends for feedback.

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