Our procedures apply to any theoretically-guided testing of interactions using regression analysis where the cross-over point is at issue. In the context of GXE interactions, some have argued that negative emotionality, a quantitative temperament factor, is a diathesis, whereas others see it as a more general malleability marker (Belsky, 1997, 2005; Belsky & Pluess, 2009; Boyce & Ellis, 2005; Ellis et al., 2011). Thus, a researcher could use quantitative measures of both the environment (X1 = childcare quality) and a genetically-related factor (e.g., X2 = negative emotionality) to test competing trends in linear X linear GXE interactions.
