that tolerance was the global norm. another reason is that lactose intolerance tends to be under-reported: lactose intolerant individuals can tolerate at least some lactose before they show symptoms, and their symptoms differ in severity. the large majority of people are able to digest some quantity of milk, for example in tea or coffee, without developing any adverse effects. fermented dairy products, such as cheese, also contain significantly less lactose than plain milk. therefore, in societies where tolerance is the norm, many lactose intolerant people who consume only small amounts of dairy, or have only mild symptoms, may be unaware that they cannot digest lactose. eventually, in the 1960s, it was recognised that lactose intolerance was correlated with race in the united states. subsequent research revealed that lactose intolerance was more common globally than tolerance, and that the variation was due to genetic differences, not an adaptation to cultural practices. other animals most mammals normally cease to produce lactase and become lactose intolerant after weaning. the downregulation of lactase expression in mice could be attributed to the accumulation of dna methylation in the lct gene and the adjacent mcm6 gene.