Conceptual model. Figure 1 depicts the conceptual model of the study. It shows the cross-sectional and longitudinal relationships (in the time span between age 18 months and 36 months) between behavior problems and BMI which are to be examined. The cross-sectional relations correspond to correlations and are represented by bowed two-sided arrows. The longitudinal relations correspond to regressions and are represented by straight one-sided arrows. Both behavior problems and weight are expected to be associated over time, indicating relative stability. The cross-lagged pathways are depicted by the crossing arrows. All relationships are controlled for relevant confounding variables (background variables).