What actual evidence is there, for or against, towards somatotypes on a physical level.
There are lots of great answers regarding this topic on this very site:
I have my own theory regarding this topic that I've developed over the years by reading a lot of nutrition articles and journals. Although there is research in this field, from my point of view it all boils down to insulin and leptin sensitivity.
I describe the effects of leptin and insulin here:
Basically, I developed this theory over the years after experimenting on myself. As a kid, I used to have a very "sugary" diet. I'm talking cake and chocolate milk for breakfast, a jar of nutella for a snack, and a bunch of other high-glycemic foods like white rice and pasta for lunch and dinner, with barely any physical activity. Now, if an individual does this for some time, they become insulin resistant (and hence leptin resistant). Now, in VERY basic terms, this equates to an "endomorph"...this person will find it really difficult to lose excess fat compared to the other groups, since insulin resistance will make it so that this person's body won't utilize carbs effectively and consequently it will be easier for this person to add fat. Now, obviously this doesn't have to be your childhood diet, I'm just giving you a specific case for which this happened to me and I was able to fix it through years of proper diet, and am no longer an "endomorph". Similarly, anyone with a diet high in simple carbohydrates (high GI) will end up like this over time.
An obviously, a person who follows a healthier diet (not even necessarily healthy, just lower in high GI foods), does more physical activity, has a higher BMI and thus a higher metabolism usually gets grouped to one of the other two somatypes.
Genetics DO play a role however. For example, people have different metabolic rates with different genetics, different muscle mass, different activity levels. So it's important to note this.
I've seen so often in the past that a "skinny" person will eat whatever they want and not gain any weight, whereas a "normal to overweight" person will eat a bag of chips and instantly turn into a balloon. What most people don't realize about this scenario is that the "ectomorph" most likely has better insulin sensitivity that he has developed over years of a relatively healthier diet, and now he's seeing the "benefits". Furthermore, while this person may be skinny, he may actually have more muscle mass relative to his body than the "endomorph" and thus a higher metabolic rate.
Just what I've observed over the years.