In 1966, the historian Cecil Roth questioned the inclusion of all Yiddish speaking Jews as Ashkenazim in descent, suggesting that upon the arrival of Ashkenazi Jews from Central Europe to Eastern Europe, from the Middle Ages to the 16th century, there were already a substantial number of Jews there who later abandoned their original culture in favor of Ashkenazi culture.[2][3] However, according to more recent research, mass migrations of Yiddish-speaking Ashkenazi Jews occurred to Eastern Europe from the west who increased due to high birth rates and absorbed and/or largely replaced the preceding non-Ashkenazi Jewish groups of Eastern Europe (the latter groups' numbers are estimated by demographer Sergio DellaPergola to have been small).