I think you'll find that there is substantial evidence of an association between spatial (or visuospatial) abilities and mathematical competence. However, most studies will focus on schoolchildren and most of the rest on adults in school (e.g., college students) or those with acquired brain lesions.

Psychological research on healthy, non-literate adults is quite scarce. To begin with, such people are difficult to find and recruit in the developed world - where virtually all psychologists are located. 

I am attaching one possibly relevant document. The author, Samar Zabar, is now (I think) at the American University in Beirut. Try  [email protected]  to contact him - but I'm not sure he's still working on this topic.

You also might want to check out the literature on developmental economics. I know, for example, that Chris Udry at Yale had planned to include a couple of cognitive tasks in his research in Ghana (probably a partial Raven and a digit span test, which would be pretty good for your purposes). You can find some of his working papers and data at the attached site; it will also connect you to other developmental economics sites.

If you're serious about this, it could be an excellent idea for a research program! Because very little previous work bears on the specific issue, yet the broader context is well established.

http://www.econ.yale.edu/~cru2/

Thanks for the excellent references, Steven! Yes, I am serious (I've been working on this for years, but not with academic credentials) and have published several papers on my own tests which are available here. If you are interested in collaborating on this program, I am certainly looking for such. 

Warm regards,

Brett

Full studies on illiterate or even underliterate adults are rare, and tend to focus on reading skills, but there are some interesting specialized areas that are related to your question:

One line of inquiry could be contact with isolated tribes. There is a famous study from about 10 years ago with the Pirahã in Brazil (Frank, Everett, Federenko, & Gibson, 2008). Their conclusion is around linguistic determinism, but it has some interesting connections about number cognition.

Another is the area of comparitive psychology. Furlong and Opfer (2007) found that chimps behave a lot like children on some number tasks.

http://www.sciencedirect.com/science/article/pii/S0010027708001042

Article Numbers, not value, motivate cooperation in humans and orangutans

Dear Brett Mathews,

The bulk of existing research on the positive correlation between spatial training, spatial reasoning and mathematical competence has been on the rise involving different age groups and contexts.It is assumed that the relationship between space and math may be based on common underlying processes which suggest a promising avenue for mathematical improvement. Notably, spatial reasoning is a malleable skill that can be improved with training. As you have rightly observed, math is a symbolic language which enables humans to think about the quantity of matters while relating thoughts and the relations between quantities to each other. For more information, I refer you to the following links which I hope are beneficial.

Best regards,

R. Biria

https://www.sciencedaily.com/releases/2016/08/160817131930.htm

https://files.eric.ed.gov/fulltext/ED528883.pdf

https://www.dartmouth.edu/.../Casey,%20Dearing,%20Vasilyeva,%20Ganley,%20and%20Ti..

Dear Brett

You ask if there are studies linking spatial reasoning skills with numeric cognition in unschooled/illiterate adults.

Let me start by saying that both spatial reasoning and mathematical competencies are forms of cognition, spatial cognition and logical-mathematical cognition, respectively. As the individual who thinks in terms of physical space is the same who thinks in mathematical terms, it seems to be empirically evident that it exists an association between spatial reasoning and abilities and mathematical competences and reasoning. As referred to Stephen Joy, there is in fact substantial evidence of an association between spatial abilities and mathematical competence, namely in schoolchildren and adults in school  

This finding is not surprising. For example, if one adopts a Piagetian structural and constructivist perspective, that one's knowledge is not a mere compilation of facts but rather a constructed and structured whole, the above mentioned association would be predicted, for example, from Piaget' and even Bruner' s theories of the individual's cognitive development. Note that Piaget found that children who conceive of space in Euclidian terms are also capable, for example, of number conservation, or the other way around. This correspondence suggests that common psychological and neural processes are at issue in both competencies. Lo look for these processes would be to perform an interesting research

However, you ask for studies linking spatial reasoning skills with numeric cognition in unschooled/illiterate adults. I know of no study related to this issue, but see previous answers to your question.

However, if there is substantial evidence of an association between spatial abilities and mathematical competence in school children and adults in school (e.g., college students) why, we might ask, that this is not the case in unschooled/illiterate adults?

You wonder whether there is some evidence that spatial reasoning improves arithmetic skills in young children. As it is this, your concern seems to imply that you admit that spatial reasoning improves arithmetic skills in young children. Why not also admit that mathematical reasoning improves spatial skills in young children?

I hope I have got your question and that this helps.