reinvesting the rental income into new real estate purchases and repeating over and over? If it does not then I can understand the conclusion in stocks favor. Not sure purchasing one property, holding and letting it appreciate would out preform the market generally but it seems unlikely. Maybe if it is an apartment complex in a good area. I don't mean to be facetious but I really would love to read that research.

If reinvestment is not taken into account than those studies are missing one of primary steps to true real estate investment. Over the course of 20-50 years a lot of income producing real estate can be purchased this way, plus you are likely to run into several recession level buying opportunities which speed up the process significantly because of the bargains you can find. If rents and property value increase, as is normal, your real estate portfolio is very lucrative as you age, plus the value is still increasing significantly as you live off of the income. For example, I have a family member who in 2013 made a one property $18,000 investment with his personal funds followed by another $20,000 injection of personal funds on a second property 6 months later. Since then he has dedicated all rent to maintaining and building that portfolio. By using sustainable loans and only those rental proceeds he now has 10 properties bringing in total net of $2700-3000 a month, they have appraised for enough that his increase in net-worth is over 10X his investment even if he paid back all the loans. That happened in 7 years and not in a super hot market and he'll likely just continue to buy at least a property every year for the next 30 years while letting them pay themselves off. Admittedly this last recession might be an outlier for real property prices given the circumstances.

It probably takes more time than no time if you plan to never review you market funds or make any decisions about reallocating them, but with good property management that time is significantly diminished because most of what you are doing is due diligence and approving expenses.