"To internalize those behaviors" means to adopt or incorporate certain actions or attitudes into one's own belief system and to make them natural or automatic in one's daily life. Full definition
Combined effect sizes for parent and teacher ratings of internalizing behavior problems were small (< 0.28) and for externalizing behavior problems negligible (< 0.09) and not significant. (pediatrics.aappublications.org)
The prevalence of total problems was estimated as 2.9 %, the prevalence of externalizing behavior problems was 2.5 %, while the prevalence of internalizing behavior problems was 3.8 %. (scirp.org)
First grade emotion knowledge as a predictor of fifth grade self - reported internalizing behaviors in children from economically disadvantaged families (cambridge.org)