They found the majority of the height jump was during the Great Depression and the 2 world wars. They concluded that it was because of smaller families that people chose to have which caused people to grow taller ... Less mouths got more food, I guess.
I'm wondering if it wasn't more due to inter-nationality breeding - after all we can't say all the people born during that time were from marriages, and the number of out-of-wedlock births seems to rise during national crises ...
But think about it ... If you bring in Scandinavian blood where the men are naturally tall you will effect the height of the offspring ... If you brought in the blood of a nation with shorter averages, you will decrease the average height ... BUT this isn't going to happen overnight, it will take a few generations.
They have seen this in feral dogs ... You know, those mitts who roam the streets/woods/etc but don't belong to anyone but themselves ...
The first breeding between pure reeds will take on characteristics of both parents, but after several births & cross-breedings the pups begin to show drastic changes that have little to do with the original pair ... While the original parents might have been, say a boxer and a poodle, the great grandchildren will look more like a shepherd mix - longer nose, dog-earred, shorter coat in warmer climates longer coats in colder, long tail, particolored coats (multiple colors one the same coat).
While a good deal of human growth can be linked to the nutrition we take in - much like the jump in height the Japanese experienced after WW2 (children were taller than their parents) - these are usually slight but instantly noticeable increases (one or two generations) ...
They are speaking of a range of approximent ally 5 generations ... A generation is about 20 years for scientific studies, but it varies by country .. In the USA a generation is like 22.5 years right now on average ... In other countries it might be 18 years, 15 years, or even 30 years on average -- so science uses 20 years as a median generation.
For long range changes one has to look at other factors than food sources ...
It could also be linked, I suppose, to the use of chemicals & preservatives in our food ... These can stick around for several generations passed from parent to child, then from child to grandchild, etc.
Small genetic changes which are made which we have no idea about for years and years to come ... Perhaps like the sudden increase in the number of celiac disease, peanut allergies, and ADD/ADHD cases each year ...
Some of this might simply be over diagnosing (if you read my blog for a while you know I believe that many ADD/ADHD cases are simply "flavor of the week" types of diagnosises) but one can not rule out that playing genetically with food stuffs (in particular meats) might be effecting our children as well.
During the great depression and the world wars the use of vaccines & hormones were being used greatly in cows to increase the amount of meat one could get off a single cow - less grass/field needed for animals, more for veggies, plus the added benefit of more meat for people.
BUT I am just speculating here ... It's been way too long since I've done a study - and since they only looked at statics to do their measurements and get data l.l there is too much iffiness in the study to really mean anything at all ...
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