Gemara

Question: The Mishnah should say that Ervah exempts from Yibum and Chalitzah (the primary Mitzvah should be mentioned first). Alternatively, it should say only that she exempts from Chalitzah (and obviously, from Yibum)!

Answer: The Mishnah is like Aba Sha'ul, who says that Chalitzah is preferable to Yibum.

39a - Version #1: R. Yochanan and R. Yehoshua ben Levi argued about which is preferred, Yibum of a younger brother or Chalitzah of the oldest.

One prefers Yibum of the younger because the Mitzvah is to do Yibum. The other prefers Chalitzah of the oldest, for when there is an older brother Yibum of a younger brother is not the Mitzvah.

Version #2: Both agree that Yibum of a younger brother is preferred. They argue about whether Chalitzah of the oldest is better than Chalitzah of a younger brother, or if they are equivalent.

39b (Mishnah): The Mitzvah of Yibum is preferable to the Mitzvah of Chalitzah. This was initially, when people intended for the Mitzvah. Nowadays that people do not intend for the Mitzvah, Chalitzah is preferable.

Rav: We do not force (rather, they decide).

When a Yavam and Yevamah would come in front of Rav, he would say 'It is your choice. Do Chalitzah or Yibum.' "If he does not want" teaches that if he wants, he may do Chalitzah or Yibum.

Also Rav Yehudah holds that we do not force. Rav Yehudah enacted to write in a document of Chalitzah '...We told him 'If you want, do Yibum. If not, stick out your right foot.''

Mishnah: Nowadays that people do not intend for the Mitzvah, Chalitzah takes precedence.

Rami bar Chama: They returned to say that Yibum takes precedence.

Question (Rav Nachman bar Yitzchak): Did the generations improve?!

Answer: No. Initially, they held like Aba Sha'ul. Later, they held like Chachamim:

Beraisa - Aba Sha'ul: If one did Yibum for (her) beauty, or to be called a married man, or for another reason (other than the Mitzvah), it is as if he transgressed Arayos. In my eyes, the child is nearly a Mamzer;

Chachamim say "Yevamah Yavo Aleha" - for any motivation.

109a (Bar Kapara - Beraisa): A person should always cling to Chalitzah.

This is like Aba Sha'ul.

Kesuvos 64a (Shmuel): We write an Igeres Mered against an Arusah (to deduct from her Kesuvah for refusing to go to Chupah), but not against a Yevamah (who refuses to do Yibum).

Question (Beraisa): We write even against a Yevamah.

Answer: The Beraisa is like the initial Mishnah; Shmuel is like the latter version:

(Mishnah): At first, when people intended for the Mitzvah, Yibum was preferable. Nowadays, Chalitzah is preferred.

77a (Mishnah): If a husband has any of the following problems, we force him to divorce his wife: a leper,... or a tanner (he is odorous), whether it preceded the marriage or came during the marriage;

R. Meir says, regarding all of them, even if he stipulated with her, she can say 'I thought I could bear it, but now I see that I cannot.'

Chachamim say, she is forced to bear it, except for a leper, since this harms her.

A case occurred in Tzidon, and a tanner's wife fell to Yibum to a tanner. Chachamim said that she can claim 'I could bear your brother, but not you.'

Rishonim

Rif and Rosh (4:17): A Mishnah teaches that Yibum used to be preferable to Chalitzah when people intended for the Mitzvah. Now that people do not intend for the Mitzvah, Chalitzah is preferable. Rav and Rav Yehudah do not force; we let them decide. They returned to say that Yibum takes precedence, i.e. the Halachah follows Chachamim. "Yevamah Yavo Aleha" - for any motivation.

Nimukei Yosef (DH Gemara): The Ritva derives from R. Yehoshua ben Levi and R. Yochanan that Yibum is preferable.

Rebuttal (Hagahos Maimoniyos Yibum 1:1): This Sugya discusses people who intend for the Mitzvah (but normally, Chalitzah is better).

Nimukei Yosef (ibid.): The Rif and most Ge'onim rule like Chachamim. Tosfos rules like Aba Sha'ul, since the opening Mishnah or our Masechta is like him. Sometimes it says 'they retracted to say' and this is not the Halachah.

Rambam (Hilchos Yibum 1:2): If the Yavam or Yevamah does not want to do Yibum, he does Chalitzah and then she is permitted. It is a Mitzvas Aseh to do Chalitzah if he does not want to do Yibum. The Mitzvah of Yibum takes precedence over the Mitzvah of Chalitzah.

Hagahos Maimoniyos (2): Even though normally Yibum is better, the Sifri teaches that Chalitzah is better if she is sterile or old (and cannot have children).

Rambam (2:10): If a Yevamah permitted to do Yibum does not want to, she is like one who is Moredes against her husband. We force him to do Chalitzah; she does not receive her Kesuvah.

Magid Mishneh: The Rambam (Ishus 14:8) holds that we force any husband to divorce a Moredes. Whenever the Gemara says that we request, this means to also give a Kesuvah. Some say that when he is unfitting we do not write Igeres Mered. This seems correct, for in this case Yibum is not preferable.

Rosh (Yevamos 4:17): Rav does not force. Rashi explains that if both of them want to do Yibum, we let them. We do not say that because they do not intend for the Mitzvah this is like Arayos. Rav Sheshes expounds the Semichus of "Lo Sachsom" to Parashas Yibum that we do not silence a Yevamah who does not want to do Yibum with a leper. The same applies to any proper reason, e.g. he is a tanner. If he does not want to do Chalitzah, we try to trick him. E.g., we tell him to do Chalitzah and she will pay him 200 Zuz. If we cannot trick him we force him to do Chalitzah. We do not write an Igeres Mered against a Yevamah.

Rebuttal (Rosh): We cannot learn that we honor any proper reason from a leper or a tanner. Beis Din forces any such man to divorce his wife if she requests! The Gemara that says that we do not write an Igeres Mered against a Yevamah holds that Chalitzah has precedence. Rashi says that nevertheless, we do not force to do Chalitzah (unless she has a proper reason). This is wrong.

Rosh (ibid.): R. Tam explains that 'Rav did not force' refers to the Reisha, when Yibum was the primary Mitzvah. If they intended for the Mitzvah, Rav allowed them to do either. Also Rav Yehudah gave the option to people who came to do Chalitzah. If they do Yibum, surely it is l'Shem Mitzvah. However, normally we force to do Chalitzah, for the Halachah follows Aba Sha'ul. The Mishnah (brought in Yevamos 39b) is like him, and the Stam Gemara (3a and Kesuvos 64a) and Bar Kapara's Beraisa (109a). Here R. Tam ruled that Yibum is preferable. Later, he retracted.

Poskim

Shulchan Aruch (EH 165:1): The Mitzvah of Yibum has precedence over Chalitzah. If she does not want to do Yibum with any brother (Rema - or with the oldest when he wants to do Yibum) without a proper reason, she is like a Moredes. Some say that Chalitzah has precedence.

Rema: According to the latter opinion she is not like a Moredes if she does not want to do Yibum. In any case we do not force him to do Chalitzah, but we trick him into doing it if possible, e.g. we tell him to do Chalitzah on condition that she will pay him 100 Zuz. This is only if he is not a kind of man whom we force him to divorce his wife if she requests. If they want to do Yibum we do not allow them unless it is known that they intend for the Mitzvah. Some say that if he already has a wife, we force him and excommunicate him until he does Chalitzah. Some say that even if he does not have another wife, if they do not intend for the Mitzvah and she does not want to do Yibum and we cannot trick him, we force him.

Beis Shmu'el (1): According to the Poskim that Yibum is preferable, there are three laws. If we do not know that they are l'Shem Mitzvah, we allow Yibum. If we are unsure, we force verbally. If we know that they are not l'Shem Mitzvah, we force physically. This is unlike the Beis Yosef and Darchei Moshe, who connote that we do not force physically even in this case. Our custom is to be concerned for the Rif and Rambam who say that Yibum is preferable, so we do not force physically even if we know that they are not l'Shem Mitzvah. If he already has a wife all agree that Chalitzah is preferable, so we force physically. However, this is not necessarily so, for Yibum is not forbidden. Some say that when he has a wife we force even when we do not know his intent, due to R. Gershom's decree.

Question: We concluded that the Torah permits Yibum of Chayavei Lavin because we cannot fulfill both; Chalitzah is not a fulfillment of Yibum. According to those who rule that Chalitzah is preferable, how can we answer?

Answer (Beis Shmu'el 174:3): All agree that mid'Oraisa Yibum is preferable. Even though the Tana'im argue about verses, Aba Sha'ul forbids mid'Oraisa only when there is improper intent.

Chazon Ish (129:13): The Nimukei Yosef says that Aba Sha'ul never forbids mid'Oraisa. He agrees that mid'Oraisa the proper Mitzvah is Yibum with proper intent. Mid'Rabanan we prefer Chalitzah lest he have improper intent. Yibum with improper intent acquires, even if she is forbidden by a Lav. We expound that Beis Din counsels the Yavam about which is better. Chachamim agree that Chalitzah is better if there is a great age difference between them, or he is already married, or if she is Chayavei Lavin and he would have to divorce her after one Bi'ah. Nevertheless, if he did Yibum he acquired her. However, the Ramban (39b DH u'Mashma) says that according to Aba Sha'ul Yibum without intent for the Mitzvah does not acquire. If so, the Halachah does not follow Aba Sha'ul.