“Fake it ‘till you make it.”
This (well-meaning advice) is designed to help people overcome low confidence and imposter syndrome. It might work for some people, but it is counterproductive for many others.
Instead, ask what you or your people would need to be ready, willing, and able to take on a new or expanded role. Build your people up with authentic positive reinforcement (“I think you’re going to be great at this”), or with training (“Let’s make sure you know how to handle the situations that might come up”), or with support (“If you get wonky results like X, come talk to me. If Y or Z happen, bring in Sam and Pat—they know how to fix it.”). Give yourself and your people what each person needs to honestly be able to say to themselves: “I can handle this.”
Set your people up for success. Appreciate their successes and good efforts. Build them up further by being a sounding board for revisions and continual improvements. Over time, people can grow into roles of greater responsibility… and feel comfortable that they belong in those roles.
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