Ask entrepreneurs about their companies, and they answer with alacrity and specificity. Ask why they wanted to start those companies, and they grow vague: "It's in my DNA." "I have a passion for it." "No one else would hire me."

But entrepreneurs' true motivations are more nuanced than that. They are also important. Founders who understand what matters most to them are more likely to create ventures that satisfy them emotionally as well as materially, according to Noam Wasserman, an associate professor at Harvard Business School. "One of the key things about entrepreneurs is that they have far more potential to make decisions with both head and heart," says Wasserman. "When you're taking the world on your shoulders, you have to ask yourself, Why am I doing this? If you only listen to your head, the decisions you make at every fork in the road can drive you farther from your personal promised land."

Wasserman's book The Founder's Dilemmas: Anticipating and Avoiding the Pitfalls That Can Sink a Startup will be published this month. Based on his popular Harvard Business School class, Founders' Dilemmas, thebook helps aspiring entrepreneurs think through the crucial decisions aboutwhen and how to launch. It also urges people already running businesses to conduct a little soul searching about what they want from the experience.

Wasserman and Timothy Butler, senior fellow and director of career development programs at Harvard Business School, surveyed roughly 2,000 founders about their motivations. They carved out results for men and women in their 20s, 30s, and 40s-and-beyond, and compared the results with those for thousands of non-entrepreneurs who completed the same survey. Among their findings: Entrepreneurs are from Mars; non-entrepreneurs are from Venus. When respondents were asked to rank 13 motivations, entrepreneurs gave priority to things such as autonomy and power. Non-entrepreneurs, by contrast, valued security and a congenial work environment. The study also showed that motivations change with age, with women's motivations shifting more than men's.

Theoretically, entrepreneurs who anticipate shifts in their needs can structure organizations that change along with them. Of course, human beings are driven by multipledesires, and so founders have to weigh sometimes conflicting motivations when making such crucial decisions as how much equity to take, what to delegate to the senior team, and whether to sell or stick.

The motivations survey is part of a larger self-assessment tool called CareerLeader, co-developed by Butler and used by hundreds of universities and business schools around the world. Click here to play with an interactive version created for Inc. by CareerLeader, and see exactly what is driving you into the hardest and most thrilling profession in the world.