“I can’t” is not nearly as powerful as the rationale “I don’t” because it’s a mere restriction. When we say “I can’t,” we don’t dive into our identity. We instead imagine ourselves confined. “I can’t” implies if we had our choice, we would give in. “I don’t,” on the other hand, directs us to our identity—to who we are.

“I don’t” has more impact than “I can’t.”

I first came across the research behind the difference between “I can’t” vs. “I don’t” while reading The Motivation Myth by Jeff Haden (but the research has also been discussed on Forbes). Backed with the scientific studies, Haden explains,

Stop saying “can’t” and start saying “don’t.” It works. Science says so. Researchers conducted a study: One group was given a simple temptation and told to say, in the face of that temptation, “I can’t do (that).” The other group was told to say, “I don’t do (that).” What happened? Participants told to say “I can’t” gave in to the temptation 61 percent of the time. Participants told to say “I don’t” gave in to the temptation 36 percent of the time. (Jeff Haden, The Motivation Myth, 91).

In this first study, “I don’t” was found nearly twice as effective as “I can’t.”. But the research becomes even more convincing, as Haden continues,

The same researchers conducted a further experiment: Participants were told to set a personal long term health and wellness goal…One group was told to say, “I can’t miss my workout.” Another group was told to say, “I don’t miss my workouts.” (The control group was not given a temptation avoidance strategy.) Ten days later the researchers found: Three out of ten control group members stuck to their goal. One out of ten “I can’t” group members stuck to their goal. Eight out of ten “I don’t” group members stuck to their goal…Why? According to the researchers, “The refusal frame ‘I don’t’ is more persuasive than the refusal frame ‘I can’t’ because the former connotes conviction to a higher degree.” (91-92).

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