That people respond to incentives is an obvious point but I feel like every reiteration is worth it. One of the clearest examples of where people respond strongly to incentives is retirement. If you raise the retirement age, many people who'd otherwise be eligible continue to work. Since retirement probably increases life satisfaction/happiness and perhaps even health we obviously want it to happen at some point, but since it's also very costly in terms of benefits paid and productive activity not done, we want to be mindful of both costs and benefits.
A new paper in The Review of Economics and Statistics by Kadir Atalay and Garry F. Barrett at the University of Sydney adds to a large literature:
Governments around the world are reforming their social security systems in light of the challenges posed by population aging. We study the 1993 Australian Age Pension reform, which progressively increased the eligibility age for women from 60 to 65 years. We find economically significant responses to the reform. An increase in the eligibility age of one year induced a decline in the probability of retirement by 12 to 19 percentage points. In addition, the reform induced significant program substitution, with increases in enrollment in other social insurance programs, particularly the disability support pension, which effectively functioned as an alternative source of retirement income.
Raising the retirement age for women led to lots more of them working, but also more of them claiming other benefits.
Every single paper I've ever seen on the topic has found a similar result. For example in the UK raising the pension age from 60 to 61 led to 7.3pp more women in employment at age 60 (separate paper with more evidence). In Spain, people with worse health were more responsive to financial incentives. Less generous pension payouts in France (normal retirement rather than disability insurance retirement) meant 14% higher total work hours, on average, between the ages of 55 and 64. Another paper found that pensioners respond to incentives in a different way: if they stand to gain more by waiting before they claim then they are more likely to wait.
The point of all this is not to say that we should pack the elderly off to the workhouse until they're 90, but more to note that incentives matter, against the common claims that the homo economicus model is rarely or never a good approximation for real humans.