--- - |2 An empirical model of temperament that assessed transactional and cascade associations between respiratory sinus arrhythmia (RSA), negative affectivity, and the caregiving environment (i.e., maternal intrusiveness) across three time points during infancy (N = 388) was examined. Negative affectivity at 5 months was associated positively with maternal intrusiveness at 10 months, which in turn predicted increased negative affectivity at 24 months. RSA at 5 months was associated positively with negative affectivity at 10 months, which subsequently predicted greater RSA at 24 months. Finally, greater RSA at 5 months predicted greater negative affectivity at 10 months, which in turn predicted greater maternal intrusiveness at 24 months. Results are discussed from a biopsychosocial perspective of development. - Child Development, Volume 89, Issue 5, Page 1735-1751, September/October 2018.