You may have some idea in your mind that things like cancer, intelligence, and obesity are in some way caused by genetics. By the time you are done reading this post, I hope you will no longer labour under this gross misconception.
First off, repeat after me: correlation is not causation. Go ahead, say it out loud. You will make the same mistake again anyway, I guarantee it. Saying it a couple times will help you make this mistake less frequently.
The canonical example is that intelligence is correlated with foot size. It's a fact. If you take a random sample of people and issue them with whatever kind of test you like, you will find that there is a correlation with test score and foot size. What this means is that on average a person with larger feet is more likely to have a better score on the test. In case you haven't figured out why this is yet, consider the case of those individuals whose feet are just a few inches long. They don't score very well on many tests do they. Infants can't even walk! That's right, what's going on here is that older children are more likely to score well on tests, and at the same time they are more likely to have bigger feet. So, lets go ahead and draw the incorrect conclusion from our study shall we?
Intelligence is caused by big feet.
This example is nice because you intuitively realize that the foot is not a crucial element of intelligence. Perhaps you know of someone who has lost a foot or two and was still able to put some sentences together. Perhaps you know something about the nervous system and the brain. Whatever the case, you brought additional knowledge to the table that you were able to use in assessing our theorem and despite the hard factual evidence of the correlation, which by the way most definitely is evidence in support of the theorem, you discarded the theorem as false. Unfortunately, it is not always so easy.
Lets consider another example. Studies have shown that Chinese people score better on average than other people on spatial reasoning tests. This evidence supports the following theorem:
Han genetics causes superior spatial reasoning intelligence.
What do you think? Presuming you accept the study at face value, as I do, you accept that Han genetics is correlated with spatial intelligence. Do you therefore agree with the theorem? Here's another bit of evidence that might help your decision: Chinese people have spent on average tens of thousands of hours studying and memorizing thousands of spatially arranged symbols. Yeah, that might have something to do with it. Do you think an illiterate North American born person who happens to have Han genetics but speaks only Spanish will have better than average spatial reasoning abilities? I doubt it.
OK, lets get on to obesity and cancer. Studies have shown that if you are obese, it is more likely than average that your genetic siblings, children, and parents are obese. Gosh how could this be? It must be that obesity is genetic right? Other studies have shown that your risk of getting cancer is correlated with the amount of cancer your parents and grandparents have had. Wow, cancer is genetic too. Because correlation is causation.
Cancer and Obesity are genetic.
Wait a minute. Here's a crazy idea. Do you think obesity might have something to do with what you eat? Uh, yeah. Do you think maybe families might share similar diets and lifestyles? Uh, yeah. Not always of course, but correlations are about averages. On average, a person is likely to eat similar stuff to what his genetic parents ate, because we tend to like Grandma's apple pie and Grandpa's cigarettes. Sometimes we even grew up and lived our lives in the same polluted locations, fancy that.
This is fun, lets generate some more false theorems.
Some informal studies have shown that African Americans have a better sense of rhythm than other residents of North and South America. One might find that in fact people's ability to keep a beat is correlated with the amount of melanin in the skin.
The genes that regulate melanin also enable rhythmic competence.
Do you think a talented young black drummer should put credence in my theorem, or should they thank the people that played them Elmore James and J.B. Lenoir records, encouraged them to dance, and sang songs with them when they were little? News flash: Culture is not genetic!
Since people love to pretend racist bullshit is important, I'll throw one more in here. Some studies have shown that Jewish people score better on some intelligence tests than, who knows I never actually read this shite, Somalis or something. Maybe Mormon US citizens. Jewish people are not only on average better at Hebrew, but also score better on various "intelligence tests". Well I for one believe that such tests exist which would give this result, and there may even be some people seriously dumb enough to carry out this study. The correlation will be found, and hence the theorem:
Jewish genetics cause superior intellect
My trouble with this kind of thing is not just that those believing it and preaching it are declaring their own stupidity. Clearly, anybody who believes these theorems is an idiot, but that doesn't mean it should piss me off. What pisses me off is that in believing these kind of theorems you are insulting your mothers. You are insulting your culture by not giving it the credit it is due. You are avoiding your responsibilities to pass on the parts of the culture you like, and to squash those you dislike.
Anyway, correlation is not causation.