Both papers are framed around the problem of externalities. Wikipedia defines externality as "a cost or benefit which results from an activity or transaction and which affects an otherwise uninvolved party who did not choose to incur that cost or benefit." I thought everyone else defined externality the same way. Not so. Falk and Szech include "detrimental working conditions for workers" and "child labor" as examples of negative externalities. They are not, according to the common definition, because "workers" and "children" accept the costs of working in exchange for wages. And Dasgupta and Ehrlich include fosterage, which is the practice, common in west Africa, of rearing the children of relatives and friends. Fosterage is not a negative externality of human reproduction, according to the common definition, because foster parents voluntarily accept rearing the children - they ask for it, actually.
The level of an activity that generates negative externalities is higher than is socially optimal. Thus, following their incorrect depiction of fosterage, Dasgupta and Ehrlich claim that there is excessive human reproduction in west Africa. Tellingly, they fail to mention the government subsidies to schooling and children health care in the USA and elsewhere that do contribute to excessive reproduction. And they amply discuss that each person, by degrading productive resources and polluting the environment, inflicts negative externalities on everybody else, but not the fact that people also generate positive externalities, for example by creating new resources for everyone to use productively. After adding up negative externalities and ignoring positive ones, they conclude that there are too many people around. They suggest that Africans spend less on "expensive wedding ceremonies and birth celebrations" and more on condoms ("the unmet need for family planning is substantial").
But they fail to mention that keeping mice in an appropriate, enriched environment for two years generates negative externalities. Other organisms will be killed to feed the mice. Subjects who decide to save the taxpayers who fund Falk and Szech's research ten euros are making them pay the expenses of keeping the mouse alive for two years, and are harming the unwilling organisms used to feed it. Furthermore, they are generating environmental externalities associated with rearing those other organisms. Whether this is morally preferable to killing the mouse right away is not clear to me. Is it morally worse to kill the mouse than it is to kill the other organisms? Is having more mice and less tropical forest morally better than the opposite is? Would Dasgupta and Ehrlich, who think that there are too many humans, choose to let the mouse enjoy the gift of life for two more years, at the expense of the lives of other organisms, or would they choose the ten euros?