"The relatively high proportion of Jewish slaveholding was a function of the concentration of Jews in cities and towns, not of their descent or religion"
1
Judaism causes increased likelihood of choosing to own slaves & also increased chance of choosing to live in cities
2
Living in cities causes increase chance of choosing to own slaves for all people both Jews & non-Jews & being Jewish increases likelihood to choose to live in cities
3 Other explain in comments
Treat likelihood and chance as the same meaning in this context
I am not going to rewrite it for that