preference, but if he refuses, any brother can perform yibbum, and if a brother performed yibbum out of turn, it is nevertheless valid. After one brother performs yibbum or halizah, none of the brothers may marry any of the other widows. Other laws Because there is a general prohibition on a man marrying his brother's wife, anytime that a yibbum is not required (for example the deceased had a child) it is forbidden. Likewise, anytime that there is a doubt whether yibbum is required, it is also forbidden and halizah is required. The Samaritans followed a slightly different course, which may indicate an earlier custom among the Hebrews; the former practiced the levirate only when the woman was betrothed and the marriage had not been consummated. The Karaites appear to have followed the same practice, and Benjamin Nahawendi, as well as Elijah Bashyazi, favored it. History The rabbis in the time of the mishnah added formal marriage requirements such as a ketubah (marriage contract), but over the centuries yibbum waned in favor. By Talmudic times the practice of levirate marriage was deemed secondary in preference to halizah by some of the rabbis, because of the brother's questionable intentions; indeed, to marry a brother's widow for her beauty was regarded by Abba Saul as equivalent to incest. Bar Kappara also recommends halizah. A difference of opinion appears among the later authorities, with Isaac Alfasi, Maimonides, and the Spanish school generally upholding the custom, while Rabbeinu Tam and the Northern school prefer halizah. A change of religion on the part of the surviving brother does not affect the obligation of the levirate, or its alternative, the halizah. Additionally, if the surviving brother is married, Ashkenazim, who follow the takkanah of Gershom ben Judah abolishing polygamy, would be compelled to perform halizah. Today Yibbum is a rare occurrence among Jewish communities. Orthodox Jews in modern times have generally upheld the position of Rabbeinu Tam and perform halizah rather than yibbum. Yemenite Jews, though orthodox, practiced yibbum until the en masse Aliyah of Jews to Eretz Israel in the last century. Conservative Judaism formally retains it. Reform Judaism and Reconstructionist Judaism have abolished it. Social consequences The rules may create social problems in some situations, especially for the widow. For example, among observant Jews, if the brother on whom the yibbum obligation falls is too young to marry, the widow would need to wait