> household income is a predictor of both: > - inability to delay gratification, and

I'm not sure we can draw that conclusion. Household income is a predictor of higher education -- that is well established -- and higher education as a child could mean you are more likely to have learned lessons on the benefits of waiting vs instant gratification (the principle behind savings and investments).

So higher education _might_ be correlated with delayed gratification, but not household income itself.