More work needs to be done before anybody can say anything definitive about lactose intolerance, let alone that it is purely or even mainly psychologic in origin. These researchers read all those articles and decided to put them once again to the test and see what happened. There were actually three experiments, each using a subgroup of the original 16. 5 subjects tried whole milk, skim milk, chocolate milk, acidophilus milk, and milk with cornflakes. No significant differences. In simpler language, this study tended to confirm the studies I cited.
Chocolate milk is better tolerated than white milk and adding solids to milk promotes tolerance.
The subjects knew exactly what they were having, and that could easily affect the results. And the sample sizes here are even smaller, much too small to be meaningful, even if the results do correlate with previous studies. No differences in severity of diarrhea were noted after any of the five doses. Note that without the buffering effects of the meal used in the Suarez experiment, people started to get symptoms after 12 grams of milk. The small number of subjects, as always, is a real problem.