Alternatively, ability to delay gratification could be heritable and/or influenced by parenting styles, in which case controlling for household income doesn't make sense. Parents with high time preference tend to not do as well financially AND to have children with high time preference. I have no idea how much these factors are at play, everything is at least a little bit nature and at least a little bit nuture. But science must find out rather than acting squeamish. We know that patience can be improved through training and maybe eventually we will even have meds like we have for ADHD. If these things matter, we could improve millions of lives.