Later studies showed that Spearman's hypothesis is not confined to black-white differences, nor is it merely an American phenomenon. A positive correlation between score gap size and "g" loading has been observed in comparisons of European populations with Roma communities, Central Asian populations, Native Americans, and Jews (from both United States and Israel). Spearman's hypothesis has also been observed in academic achievement tests, which indicates that it is not a unique characteristic of intelligence tests. Others have used Jensen's method of correlated vectors to test whether other group differences are related to a test's g-loading, such as differences between test-training groups and non-training groups, between Protestants and Catholics in Europe, between cohorts, and so on.