Research into the question, "which programming languages are more productive?" certainly suffers from looking at single causes in a massively polycausal system.

Research into the question, "which programming languages are more productive?" certainly suffers from looking at single causes in a massively polycausal system. If you assume the effects of individual causes combine linearly, you can still look at one cause at a time. But programming languages interact with the problem domain, library availability, team preference and experience in non-linear ways.