Now, I'm sure you've heard lots of theories for the Industrial/Scientific Revolution & Great Divergence - the rich outbreeding the poor, Christianity (1500 years later) sparking learning, the Black Death, the revival of Greek learning, lots of warring European states, but I'll bet you haven't heard this one: it's all thanks to... iodine supplementation.

And what's more, this predicts the rise of the great nation of India to its rightful place as a global superpower.

"What? But Gwern, I was sure that iodine supplementation in salt, as great as it is (http://gwern.net/Iodine) only became a thing in the early 1900s and is still an ongoing project. How could that possibly explain an uptick in European growth rates dating back as far as the 1400s? (Also what's this Indian nationalism stuff?)"

Well, you shouldn't ask me but them:

"Iodine and the renaissance: Will history turn full cycle?", Kaira et al 2014:

"In this editorial, we hypothesize a possible endocrine reason to help explain Europe's march to global dominance. We suggest iodine repletion achieved by the inter-continental salt trade, as the catalyst which spurred the European awakening. As a disclaimer, we caution our readers that this is a hypothesis, and a highly speculative one at that"

They argue that salt contained much more iodine than one would guess, and so as trade picked up, people got smarter and less apathetic (less cretinous, in other words)

"Yet, natural salt was a major source of iodine in earlier times. Though salt was not 'iodized' then, it contained many minerals including iodine. A precious commodity in ancient Europe, imported as rock salt from Asia, it was an expensive spice, available to few. Salt scarcity, and iodine deficiency, prevalent in central Europe, meant that cretinism was bound to be endemic. Reports and descriptions of physicians from the 16 th to 18 th century portray an accurate idea of the magnitude of this problem: [5] We can assume that the situation would have been worse five centuries ago, when central and northern European inhabitants would have had no access to salt.

Many economic factors must have coalesced with each other, about a 1000 years ago, to encourage trade. The opening of trade routes with Asia and Africa would have enhanced access to rock salt, which also contains iodine, and increased consumption of iodine rich sea food. The rise in iodine consumption would have helped reduce cognitive dysfunction, so elegantly described by Mas'u di and Said ibn Ahmad, and stimulate intellectual thought. This is turn would have encouraged sharing of ideas, creation of universities, expansion of trade links, opening of new trade routes and import of more iodine-rich salt. Thus, a virtuous cycle would be set in motion, fuelled by iodine repletion, and perhaps, euthyroidism.

The iodine hypothesis is strengthened by the temporal profile of a few seminal events. The land trade of salt, sourced from India and the Sahara Desert, had opened up early in the course of history. Salt gradually become more and more freely available to Europeans. At the same, the rivers of Europe began to be used for transport and trade, allowing freshly caught iodine rich sea fish to reach the hinterland in time for consumption. Both these factors served to increase iodine intake, and fuelled the intellectual awakening of Urufa."

I'm skeptical of this. As far as I know, contemporary iodine surveys indicate that consuming non-iodized salt leads to iodine deficiency if not the more extreme forms like goiters; those people are consuming plenty of salt (certainly more than the Amazonians mentioned) but it's still not enough. Further, the increase in iodine deficiency is blamed in part on sea salt, so even sea salt doesn't seem to be enough. Would not a lot of that land trade of salt, presumably sourced from salt flats, be low on iodine? And the historical iodine deficiency I've read of, particularly the mountain regions, is associated with depleted soils, suggesting that trade was a minimal source of iodine. Finally, China performs its usual role as a falsifier of theories: if Europe has a lot of trade networks and active commerce, China had a lot of trade networks and active commerce and Chinese people need salt like anyone else, yet, no Renaissance/Scientific-Industrial-Revolution/Great-Divergence. Not to mention the more obvious problems: why didn't various countries with great sea/water access and accordingly high sea-salt/fish/kelp/iodine access take over the world millennia ago? Japan, Italy, Greece, Malaysia, the Caribbeans - plenty of countries where severe iodine deficiency should've been near non-existent.

Oh, and the promised superpower part:

"From an Indo-centric perspective, this editorial provides food for thought and probably sparks yet another debate. As India transitions from iodine deficiency to iodine-replete status, will the same circle of history come true for this nation? Can we be optimistic enough to assume that the next century will be the era of Indian awakening and dominance? As an answer to this question, we as editors can only write: Watch this space!"

I suppose we'll have to.

(Which is not to say that iodization may not have had political impacts; I've pointed out research indicating iodine increases liberalism, and the interesting fact that iodine seems to benefit females more and that American second-wave feminism was timed right to benefit from early affected cohorts. But I think India's problems go well beyond iodization...)