The tropical belt has been observed to be widened during the last several decades. Historical tropical expansion is associated with poleward movement of the descending branches of the Hadley Cell and the subtropical dry zones. Previous work suggests both internal variability and anthropogenic forcings have contributed. Although greenhouse gases and stratospheric ozone depletion have been implicated as primary anthropogenic drivers of tropical expansion, the possible role of other drivers remains uncertain. I leverage idealized PDRMIP perturbation simulations to analyze the tropical belt width response to various individual anthropogenic forcers, including carbon dioxide, black carbon (BC), and sulfate. Absorbing aerosol (BC) drives tropical expansion, and scattering aerosol (sulfate) drives contraction. Aerosol (BC), especially from Asia, is more efficient per unit radiative forcing than greenhouse gases in driving tropical expansion, particularly in the Northern Hemisphere (NH). Tropical belt expansion (contraction) is associated with an increase (decrease) in extratropical static stability induced by absorbing (scattering) aerosol.  A simple linearized scaling attributes BC to be the largest driver of historical NH tropical widening but with relatively large uncertainty. (Zhao, X., Allen, R.J., Wood, T. and Maycock, A.C. 2020. Geophysical Research Letters)