Hi Frank, I am not sure I completely understand your questions.
Are you talking about interspecies comparisons of utility (differences)? I.e., how can we determine whether these 20 insects are happier than this one human
or (about utility differences) that giving food to 20 insects results in more additional utility than giving the food to one human?
Literature I can recommend is:
Dawkins, M.S. (1990). From an Animal’s Point of View: Motivation, Fitness, and Animal Welfare. Behavioral and Brain Sciences, 13(1), pp.1–9.
Fleurbaey, M., and Hammond P.J. (2004). Interpersonally Comparable Utility. In: Barberà, S., Hammond, P.J., and Seidl, C. (eds) Handbook of Utility Theory. Springer, Boston, MA. https://doi.org/10.1007/978-1-4020-7964-1_8
Budolfson, M. and Spears, D. (2020). Quantifying Animal Well-being and Overcoming the Challenges of Interspecies Comparisons. In: Bob Fischer (ed.), The Routledge Handbook of Animal Ethics
Roberts, K. (1997). Objective Interpersonal Comparisons of Utility. Social Choice and Welfare, vol. 14, pp. 79–96.
There is also a forum post about the topic here by Jason Schukraft.
Or are you just generally talking about Social Choice Theory?
Happy to help if this does not yet answer your question.
My question was mainly the first one. (Are 20 insects happier than one human?) Of course similar problems arise if you compare the welfare of humans. (Are 20 people whose living standard is slightly above subsistence happier than one millionaire ?)
The reason why I have chosen interspecies comparison as an example is that it is much harder to compare the welfare of members of different species. At least you can ask humans to rate their happiness on a scale from 1 to 10. Moreover, the moral consequences of different choices for the function f are potentially greater.
The forum post seems to be what I have asked for, but I need some time to read through the literature. Thank you very much!