Breaches of research integrity have gained considerable attention due to high-profile scandals involving questionable research practices by reputable scientists. These practices include plagiarism, manipulation of authorship, biased presentation of findings and misleading reports of significance. To combat such practices, policymakers tend to rely on top-down measures, mandatory ethics training and stricter regulation, despite limited evidence of their effectiveness. In this study, we investigate the occurrence and underlying factors of questionable research practices (QRPs) through an original survey of 3,005 social and medical researchers at Swedish universities. By comparing the role of the organizational culture, researchers´ norms and counter norms, and individual motivation, the study reveals that the counter norm of "Biasedness"--the opposite of universalism and skepticism--is the overall most important factor. Thus, "Biasedness" was related to 40-60% of the prevalence of the questionable practices. The analysis also reveals the contradictory impact of other elements in the organizational environment. Internal competition was positively associated with QRP prevalence, while group-level ethics discussions consistently displayed a negative association with such practices. Furthermore, in the present study items covering ethics training and policies have only a marginal impact on the prevalence of these practices. The organizational climate and normative environment have a far greater influence. Based on these findings, it is suggested that academic leaders should prioritize the creation and maintenance of an open and unbiased research environment, foster a collaborative and collegial climate, and promote bottom-up ethics discussions within and between research groups.