Northern and Southern China

One reason that China might be so low-trust is that there are groups of Chinese who are pretty different: northern and southern.

The Qin Mountains and Huai River, for example, separate China into two regions. This is the border of the January freezing-temperature isotherm and 30 inch rain isohyet. It is also the border of different language groups and the cereal dividing line between rice cultivation (S) and wheat cultivation (N).

From Wikipedia:

"Northern China is too cold and dry for rice cultivation and consists largely of flat plains, grasslands, and desert; while Southern China is warm and rainy enough for rice and consists of lush mountains cut by river valleys. Historically, these differences have led to differences in warfare during the pre-modern era, as cavalry could easily dominate the northern plains but encountered difficulties against river navies fielded in the south."