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Joseph Smith hiding plural marriages from his first wife, Emma
Doctrinal foundation of plural marriage
Introduction of plural marriage
Questions about Joseph Smith and plural marriage
Notable plural wives of Joseph Smith
Plural marriage in Utah
End of plural marriage
Joseph Smith hiding plural marriages from his first wife, Emma
Question: Did Joseph hide his plural marriages from Emma, his first wife?
Joseph did not always tell Emma immediately about some of his plural relationships
Joseph and Emma were in a complex and unique situation with regard to plural marriage—Emma had been warned by Joseph's revelation that if she refused to allow Joseph to obey the commandment he had received, he might proceed without her permission.
We also know relatively little about what Emma knew, and when she knew it. We should be cautious in assuming that the critical or anti-Mormon narrative of Joseph constantly sneaking around behind Emma's back is accurate.
Emma had periods where she accepted plural marriage, and then later rejected it
One critic of the Church claims, "Joseph Smith publicly lied about his practice of polygamy, and lied to his own wife (Emma) about the practice."  It is certainly true that Joseph did not disclose all of his plural marriages precisely when they happened. For example, he had been sealed to Emily and Eliza Partridge already, and Emma later had one of her periods of acceptance of plural marriage, on condition that she get to choose the wives.  She chose Emily and Eliza, and so they were resealed to Joseph without disclosing that they were already sealed. Emma's change of heart didn't last long, and she soon had Joseph break off contact with the girls, and expected them to renounce the covenants they had made. 
Ultimately, Joseph had to choose between obeying Emma and obeying God
There are also other examples. It's difficult to know exactly what Emma knew, and when she knew it, because she would later insist that Joseph never practiced plural marriage. So, we have to kind of piece together the evidence from fairly fragmentary sources.
Was Joseph justified in this? Well, that's a difficult question to answer. If one doesn't believe that Joseph was commanded to practice plural marriage, then the whole enterprise was probably a bad idea. If Joseph was commanded to practice plural marriage (as he repeatedly testified that he had been), then ultimately he had to choose between obeying Emma and obeying God. And, Joseph seems to have been determined to obey God.
Question: Was Emma aware of the possibility that Joseph could take additional wives even without her consent?
Emma was warned about the possibility that Joseph could take wives even without her consent
Emma was warned about the possibility that Joseph could take wives even without her consent.  The D&C 132 revelation was Joseph's written instructions on the matter, put into writing at the request of his brother Hyrum, who felt he could use it to persuade Emma that plural marriage was a true principle.  However, there's an important line in there that speaks to the circumstance in which Joseph found himself with regard to Emma:
Therefore, it shall be lawful in me, if she receive not this law, for him to receive all things whatsoever I, the Lord his God, will give unto him, because she did not believe and administer unto him according to my word; and she then becomes the transgressor; and he is exempt from the law of Sarah, who administered unto Abraham according to the law when I commanded Abraham to take Hagar to wife (DC 132:65).
The Law of Sarah: Wives were to be first taught the revelation to see if they would accept it. If they accepted it, then they elected new wives for their husband. If they rejected plural marriage, then the Lord picked wives for the man
In short, the Lord brings up something called "the Law of Sarah"--this refers to Sarah, wife of Abraham, who in order to fulfill the covenants made to Abraham, was willing to seek out another wife (Hagar) for her husband. So, the principle seems to be that wives were to be first taught the revelation, and see if they would obey. If they accepted the law of plural marriage, then they elected new wives for their husbands. If they didn’t accept plural marriage, then God elected new wives for men. The previous verse reads:
And again, verily, verily, I say unto you, if any man have a wife, who holds the keys of this power, and he teaches unto her the law of my priesthood, as pertaining to these things, then shall she believe and administer unto him, or she shall be destroyed, saith the Lord your God; for I will destroy her; for I will magnify my name upon all those who receive and abide in my law (DC 132:64
If Emma rejected the teaching, then Joseph was exempt from the Law of Sarah
Thus, Joseph (who held the keys--and the only one who did so at the time, see DC 132:7) was to teach Emma--which he did. But, ultimately, if she refused to accept the revelation, then "he is exempt from the law of Sarah"---i.e., he no longer requires her approval or acceptance.
This is a stern doctrine, and we can all probably sympathize with Emma's situation. But, it is not clear that the alternative is any better, if one believes Joseph was acting by revelation--ultimately, either a mortal's will has to trump, or God's does. So, Joseph was to teach Emma, but if she ultimately refused, then Joseph was to obey, even in the face of her disobedience. She could not choose for him.
It may be that this clause did not apply to any other situation--the scripture says that it applies to a "man...who holds the keys of this power," and only the President of the Church did or does. So, this was likely not much of a model for others; it was very much an issue just between Joseph and Emma. One can see that throughout--the whole revelation is really targeted at helping solve their problems. (Joseph F. Smith would later say that if the revelation had not been written in that context, it would have been different, and perhaps more useful in a sense.) 
We can and should have considerable sympathy for Emma, since she was in a very difficult situation
She may ultimately have taken a harder road (leaving the Church, marrying outside the Church, lying about Joseph's teaching of plural marriage, raising an illegitimate child of her second husband's as her own child, etc.) to learning the same sorts of things that plural marriage would have taught her. As Brian Hales has pointed out, she had the hardest job (in a way) because she was the only woman who was faced with a revelation from her husband commanding it:
Emma may have also confronted the fear that perhaps she was inadequate to bind Joseph's affections, leading him to desire other companions and thus introducing the possibility that he could have been deceived by those desires. None of the first wives of other polygamists would have experienced this trial, because none of the other first wives were married to the man who received the polygamy revelation. All other pluralists could hold the Prophet and his teachings responsible....unlike Emma, they could more easily dismiss the question of whether their husband's adoption of plurality was related to their own contributions to the marriage or that they were somehow deficient. 
Emma believed in Joseph as a prophet but could not bear plural marriage
On the other hand, though, we must remember that Emma had many experiences that others did not have. (When asked by some women in the midst of the plural marriage at Nauvoo if she still believed Joseph was a prophet, she replied, "Yes, but I wish to God I did not know it." ) She accompanied Joseph to retrieve the golden plates. She wrote for him during the initial translation of the Book of Mormon. She participated in sacred ordinances, and knew Joseph and his calling in an intimate way that few if any others did, and continued to insist to her death that he had been a prophet.  So, perhaps it is not surprising that she was tested in ways that few others were. And, Joseph may well have not handled it perfectly. He likely did did his best, but it was an agonizing situation without ideal options. As Richard Bushman noted:
I see their [Joseph and Emma's] relationship as tragic. She believed in him but could not bear plural marriage. He loved her but could not resist his own revelation. They were both heroic actors on a large stage trapped in terrible moral dilemmas. 
Question: What possible modern lessons can we learn from Emma and Joseph's struggle with plural marriage?
Joseph Smith: "it is quite as necessary for you to be tried [even] as Abraham and other men of God"
These observations provide perhaps the most useful lesson for the modern members, since Joseph Smith told the Twelve, soon before his death: "'You will have all kinds of trials to pass through. And it is quite as necessary for you to be tried [even] as Abraham and other men of God, God will feel after you, and He will take hold of and wrench your very heart strings, and if you cannot stand it you will not be fit for an inheritance in the Celestial Kingdom of God.' ." (Cited by John Taylor, JD 24:197).
Harold B. Lee said of this statement:
Now I want to bear testimony to you that every one of us [the Twelve] has had that kind of testing. Some of us have been tried and have been tested until our very heart strings would seem to break. I have heard of persons dying with a broken heart, and I thought that was just a sort of a poetic expression, but I learned that it could be a very real experience. I came near to that thing; but when I began to think of my own troubles, I thought of what the Apostle Paul said of the Master, "Though he were a Son, yet learned he obedience by the things which he suffered; and being made perfect, he became the author of eternal salvation unto all them that obey him" (Hebrews 5:8-9).
Don't be afraid of the testing and trials of life. Sometimes when you are going through the most severe tests, you will be nearer to God than you have any idea, for like the experience of the Master Himself in the temptation on the mount, in the Garden of Gethsemane, and on the cross at Calvary, the scriptures record, "And, behold, angels came and ministered unto him" (Matthew 4:11). Sometimes that may happen to you in the midst of your trials. 
We should not, then, judge Joseph or Emma too harshly. Who says but what we would face similar trials with as much grace as they did? And, hopefully we won't face ours in a fishbowl, like they did.
- John Dehlin, "Questions and Answers," Mormon Stories Podcast (25 June 2014).
- "I will give you two wives if you will let me choose them," – Emma to Joseph, as per "Incidents of the early life of Emily Dow Partridge," written beginning December 1876, finished 7 January 1877, BYU Special Collections; cited by Andrew F. Ehat, "Joseph Smith's Introduction of Temple Ordinances and the 1844 Mormon Succession Question," (Provo, Utah: Brigham Young University, Master's Thesis, 1981), 60.
- Todd Compton, In Sacred Loneliness: The Plural Wives of Joseph Smith (Salt Lake City: Signature Books, 1997), 409. ( Index of claims )
- Danel W. Bachman, “A Study of the Mormon Practice of Polygamy Before the Death of Joseph Smith,” (1975) (unpublished M.A. thesis, Purdue University), 164–166.
- Joseph Smith, History of The Church of Jesus Christ of Latter-day Saints, 7 volumes, edited by Brigham H. Roberts, (Salt Lake City: Deseret Book, 1957), 5:xxxiii. Volume 5 link
- Joseph F. Smith, Journal of Discourses 20:29-30.
- Hales, Joseph Smith's Polygamy Vol. 2, 136.
- Compton, In Sacred Loneliness, 251.
- Mary Audentia Smith Anderson (editor), "Memoirs of Joseph Smith III (1832–1914)," The Saints Herald (2 April 1935): 431–434.
- Richard L. Bushman, Interview with Millennial Star Blog, 14 November 2005; conveniently reprinted in Richard Lyman Bushman, On the Road With Joseph Smith: An Author's Diary (Salt Lake City, Utah: Greg Kofford Books, Ltd., 2007), 72.
- Harold B. Lee, Teachings of Harold B. Lee (Salt Lake City, Utah: Bookcraft, 1996), 192.