It's pretty hard to untie the multiple connections and ascribe specificity to brain development and lack of a decent gut microbiome.
For example, we know pretty well that humans subjected to famine/malnutrition/toxin exposure/etc. in the first two years of life, when ~70% of brain development takes place, will tend to have decreased cognitive function over their lifetime.
If being deprived of a microbiome reduces the ability of the gut to absorb nutrients, then essentially a kind of starvation could take place which reduces brain development, and as healthy brain development seems to be one of the requirements for normal social interactions with others, it could just be nutrient starvation rather than any direct neurochemical signaling from gut microbes to neurons that's messing up brain development.
For example, we know pretty well that humans subjected to famine/malnutrition/toxin exposure/etc. in the first two years of life, when ~70% of brain development takes place, will tend to have decreased cognitive function over their lifetime.
If being deprived of a microbiome reduces the ability of the gut to absorb nutrients, then essentially a kind of starvation could take place which reduces brain development, and as healthy brain development seems to be one of the requirements for normal social interactions with others, it could just be nutrient starvation rather than any direct neurochemical signaling from gut microbes to neurons that's messing up brain development.