Epigenetics At A Glance
To prove epigenetic inheritance takes place in humans, researchers have to prove that
epigenetic inheritance will be passed on to future generations, but not because of direct exposure. For example a pregnant mother who smokes will affect herself, the fetus, and the fetus's reproductive cells, but it also has to affect the 4th not-yet existent generation. If the 4th generation is affected by smoking but is itself smoking or exposed to second hand smoke, that does count (2).
Genetic changes cannot be a possibility. Genetic changes are possible in all individuals so it's hard to rule out whether the genes changed based on enviromental factors or not (2).
Most studies are done on lab animals but because of the difference between animals and humans, discoveries that are found in the animals have to be proven that it can also work with humans, which can take some time. Twins are the only ones that will result in outcomes accurate to those of the mice. Read More About Twins And Their Role In The Field Of Epigenetics Here
Challenges
Research in human epigenetic inheritance itself is a challenge because humans have long life spans, expanding the time needed to track further than one generation extremely time consuming. Humans also have greater genetic diversity, creating a challenge in the ability to rule out genetic differences.