Additional Shia doctrines
There are additional Shia doctrines besides the Theology of Twelvers and the Aspects of the Religion.
Taqiyya
The doctrine of taqiyya, or dissimulation, states that it is permissible to hide one's true religious convictions if under the threat of death or injury. Since Shias regard taqiyya as Islamically permissible, at times they have been accused of lying indiscriminately, however Shia Muslims reject this charge and argue that this would be contrary to the laws regulating its use. The Shia conclude that the practice of taqiyya is condoned by the Qur'an and is regulated by the various Qur'anic verses that refer to it. The practice of taqiyya is regarded by the Shia as a natural human response especially in light of historical Shia-Sunni relations.
Nikah Mut'ah (Mut'ah marriage)
Fixed time marriage (Arabic: Nikah Mut'ah) is the second Qur'anic marriage form, a marriage for a fixed time with fixed conditions stipulated through mutual written or oral contractual agreement between the male and female participants.
The practice was instituted by the Islamic prophet Muhammad and sanctioned in the Qur'an. Shia conclude it is allowed according to Sharia. In contrast to non-Shia Muslims, Shia Muslims conclude that Mut'ah marriage was only forbidden by the caliph Umar and not by Muhammad. Shia argue that neither Umar, nor any other caliph, had the authority to ban what Muhammad permitted, so Shias judge its practice to remain legitimate.
By far the most common form of marriage among Shi'as is the Nikah (regular marriage), and abusing Mut'ah marriage to lead a promiscuous life is frowned at by Shias, quoting the Qur'anic call for chastity. The most common use of it is by couples that intend to marry permanently in the near future, something akin to the Western practice of engagement.