Hard to check the validity of some of the studies as the researchers have no way of knowing if the data was collected in a scientific manner
A lot of countries and continents are missing including Africa and much of Asia
Imbalance of studies used, with so many for the US that those results are also the mean for the whole study
The Chinese study only had 25 infants, so a tiny sample representing around 20% of the global population
Takahashi found that Japanese infants were often seen as Type C as they were rarely left by their PCG and so the strange situation was terrifying for them, raising the question of whether the strange situation is valid elsewhere