So most of the study is based on a formula called, Marginal Value of Public Funds (MVPF) which leads to some interesting comparisons.

Looks like thinking about the children in this case, is actually the best use of money.

We find the highest MVPFs for direct investments in the health and education of low-income children. This includes Medicaid expansions, childhood education spending, and expenditures on college. In many cases, these policies actually pay for themselves in the long-run. Children pay back the initial cost as adults through additional tax revenue and reduced transfer payments. For example, we examine four major health insurance expansions to children over the last 50 years. We calculate an average across those policies and find that for each $1 of initial expenditure they repaid $1.78 back to the government in the long-run. In particular, we find that three of four policies fully repaid their initial costs.

We find high MVPFs for policies targeting children throughout childhood. We do find high MVPFs for early childhood education programs, including an MVPF of roughly 44 for Perry Preschool and 12 for Abecedarian. In addition, we find large MVPFs for policies targeting older children, such as historical equalizations in K-12 school financing (studied in Jackson et al. (2016)) and policies increasing college attainment. Our broad patterns contrast with the notion that opportunities for high-return investment in children decline rapidly with age (Heckman, 2006).

In one case they found a 44 to 1 payoff on every dollar invested into the programs.

Programs explicitly for adults on the other hand have much more mixed results

Our results show lower MVPFs for policies targeted to adults. Most of these MVPFs lie between 0.5 and 2.