Oh, this is a slightly different topic. Of course if your belly is growling money will motivate you a lot, and that motivation tends to go down the more you go past your comfort baseline.
But think of pay disparity for example. Maybe your belly is full but your slacker colleague is making more money than you. That has a good chance of demotivating you, making you ask "why should I try harder?". Reducing this disparity and (perceived) inequity, eliminating the demotivating factor, goes a long way and has a far longer lasting effect than just the same extra money would bring otherwise.
And pay disparity can be a problem, especially if salary data is hidden. I think most of the time the problem is a lack of openness about salaries and why they are as they are.
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2. Punished by Rewards, Alfie Kohn, 1999, https://www.alfiekohn.org/punished-rewards/
3. "Intrinsic and Extrinsic Motivation ", Roland Bénabou & Jean Tirole, 2003, https://academic.oup.com/restud/article-abstract/70/3/489/15...
4. Intrinsic and Extrinsic Motivation - The Search for Optimal Motivation and Performance, Carol Sansone & Judith Harackiewicz, 2000 https://www.amazon.com/Intrinsic-Extrinsic-Motivation-Perfor...
5. "Intrinsic and Extrinsic Motivation", Lisa Legault, 2016, [Available online] http://www.lcwu.edu.pk/ocd/cfiles/Professional%20Studies/FC/...
6. "Do intrinsic and extrinsic motivation relate differently to employee outcomes?", Kuvaas et al., 2017, [Available online] https://www.researchgate.net/profile/Bard_Kuvaas/publication...
EDIT: added more recent sources