Compliance with a small request (a metaphorical foot-in-the-door) promotes compliance with a subsequent big request. Whereas some explanations expect a drop in the behavioural costs of the big request, others suspect that the effect comes from boosting the underlying attitude. However, evidence for both explanations is equivocal and circumstantial, at best. Drawing on what Kaiser et al. (2010) call the Campbell paradigm, we present an integrative account: Compliance with any request demands a corresponding attitude to counterbalance the costs of the request. In our research, 229 participants were randomly assigned to either a foot-in-the-door (i.e., initially asked to sign a pro-environmental petition) or a control condition. Small-request-compliant participants were more likely than control participants to also comply with the big request and to continue filling out environmental-issues-related questionnaires. However, this foot-in-the-door effect occurred without diminishing behavioural costs or increasing attitude levels. Accordingly, the greater likelihood of small-request-compliant participants to also comply with the big request can be parsimoniously explained by baseline variability in people's attitude levels that manifests in their compliance with the initial request. We conclude that several of the foot-in-the-door effects reported in the literature carry the risk of representing mere pseudo-effects.