When developing behavior change interventions in a systematic way, an important step is to first identify determinants relevant to a target behavior. Subsequently, limited resources create a need to select those determinants that are most relevant to this behavior. Regression analyses are commonly used for selecting determinants, but the aim of this article is to explain why regression analyses are not suitable for this purpose (i.e., the regression trap). This aim is achieved in three ways. First, by providing a theoretical rationale based on overlap between determinants. The meaning of regression weights is commonly explained as expressing the association between a determinant and a target behavior 'holding all other predictors constant.' We explain that this often boils down to 'neglecting a part of human psychology.' Second, by providing mathematical proof based on the formulas that are used to calculate regression weights. We demonstrate that the interpretation of regression weights is distorted by correlations between determinants. Third, by providing examples based on real-world data to demonstrate the impact this has in practice. This results in interventions targeting determinants that are less relevant and, thereby, have less impact on behavior change. In closing, we discuss a possible solution to circumvent the regression trap.