Have you looked for research? https://sites.google.com/site/stefanhanenberg/ https://sites.google.com/site/stefanhanenberg/ https://link.springer.com/article/1

Have you looked for research? https://sites.google.com/site/stefanhanenberg/ https://sites.google.com/site/stefanhanenberg/ https://link.springer.com/article/10.1007%2Fs10664-013-9289-1 https://link.springer.com/article/10.1007%2Fs10664-013-9289-... The above papers are not about FP but about static type systems (there tends to be lots of crossover), but I think you'll find that what it really comes down to is guarantees provided by a language. I would be surprised if similar research that focused solely on purity did not exist. This is an incredibly difficult area to study. "Better software" is a difficult metric, and even a more meaningful metric is incredibly hard to measure. For a field as young as CS it's no wonder that the research into this is still immature, so we largely have to rely on anecdotal evidence and common sense. So yeah we have some research and we have some basic common sense and we have people demonstrating through their projects that it has worked well for them. More to the authors point - what they are really talking about here is modularization being a benefit to a moving codebase. Their quotes, their 'argument from authority', is mostly about modularization. Their argument is that functional programming lends itself to modularization by limiting state space and focusing on composition.