I've never been one to shy away from multiple interests--I know what it's like to feel an obligation toward two (or more) areas. But how many hats can you try to wear before it all falls apart, you're stretched too thin, and you're not really making a difference in any of the areas you picked?

The question attacks our inclination to be impressed every time a CEO or other leader has a mile-long list of current initiatives, involvements, or associations. That inclination might not be founded in any evidence of real result, but rather the cultural idea that more is always better, and that gritty, talented people always go after good opportunities.

Except more isn't always better, as the story of Elon Musk's misery in 100-plus-hour weeks teaches us. Sure, there are admittedly dozens of serial entrepreneurs out there we can credit with multiple businesses. But their successes are often sequential rather than simultaneous--they establish one or two projects and then hand off responsibilities or sell the businesses entirely, and they start something new as their creativity and curiosity lead them.

So as you climb the ladder or start to get more and more requests to participate based on your experience and expertise, consider the following:

  • The research on results is clear. Beyond 50 hours of work per week, productivity starts to decline, regardless of how many irons in the fire you've got. "Big shot" leaders don't get a pass on that.
  • Participation always should be a fair trade. More than one project has gained steam because a well-known investor or other leader lent their weight to it. But projects should give you something back, whether that's the feeling of purpose, the ability to learn, or the right kind of exposure. Never accept a one-way setup.
  • Your word matters. Without it, you have no trust, and without any trust, reputation erodes. Be ruthlessly clear about expectations on any new project you commit to so that it doesn't destroy your ability to keep promises you've already made.
  • Simply being influential in an industry doesn't mean you'll be a perfect fit. No one, for example, probably would ask J.K. Rowling or Dr. Seuss to write a sequel to Silence of the Lambs. If an organization is looking at your achievements without also considering whether you'll match their vision and philosophies, pass.
  • Support networks work wonders. The more people you have in your life, the more time you'll need to commit to healthy relationships instead of work. But this isn't a bad thing, as they can ensure you take breaks. But on the flip side, the right people in a support network also can give you the encouragement and logistical help necessary to take on roles that otherwise wouldn't be possible.

In the end, perhaps the most important thing to consider is that taking on a role isn't just a matter of checking boxes. You technically can give the required speeches or reviews, for instance, but if people don't have time to know you and really deeply, cooperatively communicate with you, then arguably you haven't fulfilled that role to the best of your ability. So if you're not able or ready to commit to relationships, just admit it and politely decline.

Correction: An earlier version of this article incorrectly asserted that former PepsiCo CEO Indra Nooyi had joined the Amazon board of directors while she was PepsiCo CEO. She had left that job in October and joined the Amazon board in February.