If you want to get better at life, you must set goals. But goal-setting is surprisingly hard to do well.

You’ve likely heard about the importance of setting “SMART” goals—that is, Specific, Measurable, Attainable, Relevant, and Time-Bound. These are useful guidelines, but the research on goal-setting literature is clear: there’s another critical ingredient we should consider. For illumination, let’s turn to the research of Edwin Locke.

Locke is an eminent industrial/organizational psychologist. His pioneering work on goal setting beginning in the 1960s forms the foundation of much of modern goal-setting theory (his work with Gary Latham is especially influential). One of Locke’s central findings is that the most effective goals tend to be difficult.

In a well-known meta-analysis, Locke and his colleagues found that difficult, specific goals out-performed “do-your-best” goals 90% of the time.

Ninety percent!

As an explanation, he proposed that more difficult goals were fundamentally more motivating than less-difficult ones. I don’t know about you, but this has certainly been my experience. When Sarah and I paid off our debt in two years, the sheer difficulty of the plan seemed to make the whole experience more exhilarating. Locke’s research makes clear that while it’s important to set goals that are achievable, we’re more likely to feel motivated when a goal is genuinely challenging.

An aside: another key finding in Locke’s research is that the goal-striver must personally care about the goal. This seems obvious, but it’s worth lingering on for a moment. A difficult goal that you care about today must also be a goal you’ll care about three weeks from now as you’re working on it. It also must be your goal for yourself, not someone else’s goal for you.

Goal-setting is an essential part of a well-lived life. You may already be setting SMART goals, but make sure your goals are difficult as well. The anticipated feeling of satisfaction you’ll experience as you work toward the goals will help keep you plunging ahead.