This sounds like a nature vs. nurture discussion.

Nature + Nurture = Who the person grows up to be.

Genes (nature) are the functions but life experiences, environment, etc. (nurture) are the parameters.

GeneX(Twin1, ExperienceA) { return x; }

GeneX(Twin2, ExperienceB) { return y; }

The genes are like restricted scales and within each hold all possible outcomes for an individual. Some of these genes have a greater influence on the end result of an individual than others. Different life experiences, environments, etc. result in shaping each twin differently within each of their genes - since even twins cannot live identical lives and experience the same exact experiences.

Very interesting. Oddly, I've never really questioned the idea that being fat is unhealthy, probably because you hear it so much from medical authorities like doctors and public health officials. I can't remember the last time I went to the doctor without being told to lose weight to improve health conditions I don't even have (but am at, supposedly, higher risk for).

Let's consider, though, why humans can become fat. The body stores energy in the form of fat for later conversion to sugars in the event of insufficient sugars to keep the body functioning. Sugar is only part of what the body needs to keep functioning (we also need amino acids we can't synthesize except by deconstructing muscle, trace amounts of vitamins and minerals, and water), but it seems to be the most critical since our bodies are very good at storing it. To me this suggests that outright starvation is, evolutionarily speaking, a far greater threat than lack of access to particular essential comestibles. So our bodies are doing the right thing by making us fat because it protects us against the very real and serious threat of starvation.

The problem is that, today, in the first world, we have little risk of starvation. Even a lack of money is no reason to starve: charities and governments feed the destitute. But our bodies don't know this, so they keep trying to put on fat. The result is that it's very easy for people to become overweight because an ability to easily store and retrieve sugars in the form of fat was an evolutionary win: it doesn't matter how fit you appear, if you don't have enough fat you'll be the first to die of starvation when there's not enough food. So we should expect the majority of the population to be "overweight" whenever there is sufficient food available, which is exactly the trend we see (the time of real plenty has been, sadly, only in the last few decades).

So why is thin beautiful? Historically being thin was not attractive, especially for women, as evidenced by human art that has long shown women with at least some significant fat stores as the most attractive. In the first world, though, since sometime between about 1960 and 1980, thin became attractive. The reason seems clear: the middle class and even the poor could now get fat thanks to low cost high quality food, and so the rich (and others with high status) must have, probably mostly unconsciously, switched to thinking thin beautiful in order to retain a clear signal of higher status, a situation with plenty of precedent (consider the pale/tan swap, the car/horse swap, etc.). So regardless of health concerns, real or not, thin is in because it's become a means of signaling status.

Given this, we should be highly suspicious of claims that we need to become thinner. It's not a conspiracy, but it certainly looks like the usual game humans play to display status. Since our doctors are, unfortunately, also only human, they too are playing the status game and consider thin beautiful, and so are tempted to rationalize reasons why people should be thin since they want to help people and believe that they will be better if they lose weight. It's a cruel twist that humans have a very hard time losing weight.

So, what's to be done? Probably nothing, although it's a worthy goal to push the elimination of weight as a status signal because even a partial success would result in a lot less suffering for billions of people. In the mean time, at least LW readers can eliminate from themselves false beliefs that anything but extreme obesity, or extreme skinniness, has anything more than a marginal health effect.