Thinker's Blessing

Essentially, those with a Thinker's Blessing are able to follow two lines of thought at the same time. They also tend to have significantly above average soul flow rates. The trait is hereditary (both patrilineal and matrilineal), but comes at the cost of a greatly reduced fertility rate. This stems from how the Thinker's Blessing actually works- essentially, it forces twins to merge into a single body while still in the womb, allowing for two lines of consciousness to exist. However, should one's children not be twins, this process ends up killing the embryo. Triplets etc. would allow for even more lines of thought, though they're exceedingly rare.

As a disease

While greatly beneficial to those in possession of it, the Thinker's Blessing can be considered a curse in disguise to the wellbeing of a nation: those who have it are functionally near-infertile, and this consequently causes a dramatic decline in population. This has led to some classifying the 'blessing' as a Celestial virus, even.
With a Thinker's Blessing, taking several years to conceive a child becomes the norm, creating immense fragility in populations- an average woman can only bear slightly over two children in her fertile period, making true population growth almost functionally impossible.

Math (eyeballed)

Twins are 1 in 67 births in the US. Assuming that things fall apart about a month in if it's not twins, you can expect one pregnancy in 67 months, or 5 years. Multiply by 1.5 as a guesstimate for less fertile periods, and you average like, 100 months per pregnancy. So, 109 months average per child, or 9 years. Make fertility around 15-45, so ~30 years. Theoretical expected is around ~3.3 children. However, fertility goes down past ~35, fertility problems can arise with the multiple failed pregnancies, and most people won't by trying for a child at 15, so the expected number comes out to around ~2.2 for a 'career' housewife. Compare this to a woman living the same lifestyle without the Thinker's Blessing, who could easily have 10+ children in the same period with the same amount of dedication. Also note that while twins have to be conceived, only one child is born.

History

It's theorized that the Thinker's Blessing was originally a curse applied to a certain civilization. Naturally, this resulted in the civilization burning out within a few generations- potentially much less, had they experienced war. The blessing is at its highest density in what is now Whirlpool, though smatterings of it have occurred throughout the continent (one notable figure being Amaryllis of Whitewood, born all the way in the heart of the midlands).