Impactful environmental psychology needs formal theories

Changing individual and collective behavior is critical to addressing the climate and ecological crisis. Environmental psychology is thus well-positioned to contribute knowledge to guide impactful climate action. However, we argue that it is not living up to its full potential, partly because its theories often remain verbal. Formalizing theories — expressing them in the precise language of mathematics or computer code — is especially important to give substance to thinking about complex systems, where nonlinearities and feedback loops make the effect of interventions hard to predict. Formal theories increase conceptual clarity and mechanistic understanding, advance cumulative science, enable ‘ in silico ’ intervention testing, and improve integration into policy-relevant models. We illustrate how environmental psychologists can start incorporating theory formalization in their work. • Environmental psychology is dominated by theories expressed in natural language. • Such verbal theories are often too vague to enable precise predictions. • Formalizing theories has many benefits for environmental psychology. • Formal theories help to better understand, predict and influence behavior.