People perform pro-environmental behaviors not only out of intrinsic motivation, but also due to external factors such as expected social approval or financial gain. To the extent that people use their own motivations to infer the motivation of others, people may view descriptive norms favoring pro-environmental behavior as extrinsically motivated. This may in turn decrease the normative influence of the norm, as conformity can be negatively affected by perceptions that others are conforming mindlessly. While descriptive norms generally promote pro-environmental behavior change, the influential power of descriptive norms varies between studies. One possible explanation for these inconclusive findings is that people interpret others' behavior as either intrinsically- or extrinsically motivated. We propose that pro-environmental descriptive norms will be more influential when attributing others' pro-environmental behavior as intrinsically (e.g., pleasure of contributing to the environment) rather than extrinsically, motivated (e.g., fear of social disapproval). In two experiments (N = 1326), we compared participants' intention to purchase pro-environmental products between four conditions: control condition vs intrinsic norm vs extrinsic norm (Exp. 1) vs injunctive norm (Exp. 2). Results consistently found a significant increase in pro-environmental purchase intention in the intrinsic norm condition compared to both extrinsic norm condition (Exp. 2) and no-information control condition (Exp. 1 & 2). These studies highlight that attribution of behavior is vital for the adoption of pro-environmental norms.