Jinhai Yu writes for Social Science Quarterly. From the abstract:

The results of this study demonstrate that state online voter registration increases voter turnout. The difference‐in‐difference analysis shows that the states’ implementation of online voter registration increases the turnout of young voters by about 3 percentage points in presidential election years. The instrumental variable analysis shows that the usage of online registration by voters increases their turnout by about 18 to 20 percentage points.