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I have long held psychology in suspicion since it is tremendously difficult to conduct proper experiments on human beings in their natural habitats. We behave differently under different circumstances. We assume different roles in the presence of different people. There is literally no single, continuous personality that can be experimented on. You end up altering the subject the minute your test conditions and measurement mechanisms become sources of interference.

Inter-generational studies are even harder to conduct, and this difficulty poses a problem for biology as well. Given sufficient amount of time, the lineage will accumulate genetic mutations and adjust its bacterial flora in reaction to the experiment conditions. How can you test whether milk is harmful for human beings if lactose tolerance builds up over time? Your scientific mission will be in danger of turning into a tautology: Milk is harmful for lactose intolerant human beings.