In these simulated data, IQ is a confounder for the effect of income on health.
References
Van der Wal W.M. & Geskus R.B. (2011). ipw: An R Package for Inverse Probability Weighting. Journal of Statistical Software, 43(13), 1-23. tools:::Rd_expr_doi("10.18637/jss.v043.i13").