ABSTRACT We argue that the IPE literature on economic expertise has been characterised by ‘methodological elitism’ that makes it ill equipped to account for the influence of heterodox and grassroots forms of expertise. To remedy this, we posit a theoretical framework that emphasises the tensions between two forms of professional legitimacy. One is internal legitimacy and it is rooted in appeals to scientific objectivity, political neutrality and high professional barriers of entry. However, internal legitimacy may clash with external legitimacy, which is a function of a clear articulation of social purpose. Taking the surprising rise of Modern Monetary Theory as a case study, the paper makes two claims. First, its ideas did not coalesce into a distinctive heterodox brand of economics through traditional mechanisms like academic peer-review and citation networks, but through intense online activity. Second, the messaging that spurred MMT consistently combined constitutive claims with explicit declarations of social purpose. The result was an open project with a broad normative appeal, which successfully enrolled a large group of passionate followers and co-creators. To date, IPE scholars have been blind to such alternative forms of knowledge production.