Early studies of adult individuals have found a heritability of IQ between 57% and 73%,[6] with some recent studies showing heritability for IQ as high as 80%.[7] IQ goes from being weakly correlated with genetics for children, to being strongly correlated with genetics for late teens and adults.
Also another interesting tid-bit.
Meta-analyses have found that environmental factors, such as certain nutrient deficiencies, can result in large reductions in average IQ; iodine deficiency, alone, has been shown to produce a reduction of 12.5 IQ points on average.
Another
reference point:
In the early 1900s, the Great Lakes, Appalachian, and northwestern regions of the United States were endemic regions for IDD, but since the iodization of salt and other foods in the 1920s, dietary iodine levels generally have been adequate. However, sustaining these iodization programs has become a concern.
Data collected in the United States by National Health and Nutrition Examination Survey I (NHANES I) for the years 1971-1974 showed that the median urinary iodine level was 320 mcg/L, reflecting adequate dietary iodine intake. [
11] However, by the time of NHANES III (1988-1994), the median urinary iodine value had fallen to 145 mcg/L.
It's something that CDC has on their radar as well
Iodine Levels in the United States, 2000
www.cdc.gov
It dipped way down in the late 80's and early 90's, and then slowly came back up a tad by early 2000's and stabilized there. (that would cover the time period when a lot of the young adults today were in their forming years.
And keep in mind, that's just one particular nutrient. (and it can account for a loss of 12.5 IQ points).
Iodine is most certainly not alone on "nutritional impacts on IQ" list.
It may make more sense to look at the food and changing diets before prematurely scapegoating social media usage as the primary culprit.
While they're may be some impact on certain forms of cognition from social media....
Take for instance, S. Korea, Japan, and Germany...
...they have higher percentage of people on social media than we do, and they've seen either stable or increasing IQ scores.