What has never been known nor understood to any one's satisfaction is the source of anti-Semitism. Why would any large group of people be filled with hatred toward a tiny hard working, generous, peaceful minority?
I've got the answer: Every large group that is anti-Semitic is fed and driven by their own upper social class. The greater the spread between the upper class and the rest of the society, the greater the anti-Semitism. There is a linear relationship.
The reason for this connection has long been understood. The upper class is diverting the attention of the lower class from their role of class privilege by creating a scape-goat.
The societies with the greatest social class differences have been the most anti-Semitic, conversely, the ones with the least class difference are the least anti-Semitic. Linear.
This connection has been most visible to me in Scandinavia. The countries with the least visible upper class are Denmark, Sweden and Finland. The Scandinavian country I've seen with the most visible upper class is Norway.
Norway is also the home of a great deal of anti-Semitism. Check this article from the Jerusalem Post. Norway has a serious case of anti-Semitism, the other three are almost free of it.
Denmark, Sweden and Finland also have a lower gini (measured income inequality) than Norway.
All the Arab countries are wildly anti-Semitic and their upper classes are very over-the-top. The same is true of nearly all Latin American countries where part of the blame for anti-Semitism can be the Catholic Church but much more importantly, Latin America has the most class differentiated societies on the planet, and very anti-Semitic.
In Europe the countries that saved Jews were Holland and Denmark, both the most egalitarian countries. Today England has one of the worst cases of anti-Semitism and remains very upper class segregated.
On the other end of the scale, Japan has had a Jewish population for over a century but no anti-Semitism. Japan is hierarchical but has no distinct upper class. Interestingly, the best country to prove my point is our own: the United States.
Anti-Semitism in America was high from 1900 to 1955 but then went into a numeric nose dive after 1960. I've always attributed this rapid decline to the changes in the Vatican II theology but a much better explanation is the genuine disappearance of America's upper class in 1960 with the publication of the last New York Blue Book.
Summary: In highly divided societies where the upper class needs to divert attention from itself, we will find anti-Semitism...as a scapegoat. A visible and distinct upper class will protect itself by feeding a strong anti-Semitism. The degree of class difference and the level of anti-Semitism is a linear relationship.