My great, great grandmother's cause of death is listed as "Insanity" on her death record, and I'm not sure what this means. She died in Winsted, McLeod County, Minnesota in 1872 at age 37, and had just given birth to a daughter less than a month prior to her death. Could the "Insanity" have anything to do with complications from the birth? Or Postpartum depression? Or is it something else entirely?
Do you know anything about the baby? Did it survive? I would focus upon the postpartum depression angle, as it is quite real, fairly common with varying degrees of severity, and makes women do things that they might not otherwise do, but could definitely get them labelled as insane. Think about how someone might die from this affliction: a struggle with someone that caused her to be injured or killed, self harm, or possibly ending up being put in an institution and not receiving proper healthcare — therefore leading to her death.
Keep in mind that death due to childbirth or complications from childbirth were rather common at this time and would likely have been named as such.
My next angle of attack with this problem would be:
With no actual reference, I have recorded one of my individuals as Cause of Death - Insanity. My notes show it may have been recorded like that to allow a Church burial in the case of suicide. Has there been any indication of previous mental health issues or hospitalization?
Perhaps of interest to you, Pamela Pattison Lash, genealogist extraordinaire of Williams County, Ohio, fame, has some interest in the topic of insanity/mental illness in the 1800s, with a special interest in women and these conditions. You might want to contact her to learn if she knows others engaged in research about this. She blogs at "Williams County, Ohio Genealogy." She's blogged some on the topic. "Sensitive Subject--Insanity in Williams County, Ohio"; "Old newspaper--Sad account of "Wife of Andrew Svoboda, 1904."
A couple of general references follow; these may be less specific than you would like:
"A Glossary of Archaic Medial Terms, Diseases and Causes of Death," Rudy's List of Archaic Medical Terms (http://www.antiquusmorbus.com/Index.htm ); entry for "insanity," "Persistent mental disorder or derangement. No longer in scientific use; cites W. B. Saunders, Dorland's Illustrated Medical Dictionary, 2002.
"Some medical terms used in old records," Michigan Family History Network, entry for "Mania or Acute Mania," including comment that it was defined as "severe insanity"; says "Acute Mania was used as a term for death when the patient had been hospitalized..."
Perhaps someone will be able to find data that is more timely (to 1872) or more regionally relevant (Minnesota), but I did find some interesting information.
Washington (state), Third Biennial Report of the Board of Trustees of the Eastern Washington Hospital for the Insane ... (Olympia, Wash: O. C. White, state printer, 1896)
Statistics of the the United States (including mortality ...) in 1860; compiled from the original returns and being the final exhibit of the [1860 U.S. census] ... (Washington: Government Printing Office, 1866), p. 343 for "Mortality of the United States. "Insanity was stated to be the cause of death if 300 in 1850 and 452 in 1860, in all the states ... The proportion was twice as great in New England and New York as in Michigan, Wisconsin, Minnesota, and Nebraska ... The large proportion of insanity in California is produced by the excitement and oppressive anxieties, and the great and sudden changes of fortune among many of the people."
Also, a report by the Rhode Island Registrar of Vital Statistics, including p. 236, a table reporting the "Mortality in the State from Insanity, 1866 to 1903, inclusive."
Although I did not much review this material, it might provide even more detail on the topic, thought also from a later time. See, Henry C. Chapman, M.D., A manual of Medical Jurisprudence: Insanity and Toxicology (Philadelphia: W. B. Saunders & Company, 1903).
As other answers have noted, the meaning of the term (insanity) has not been fixed and was subject to significant variations even at one time.
For a contemporaneous opinion, you could look at Thoughts on the causation of insanity by J P Gray From the American Journal of Insanity for October, 1872
Available from http://archive.org/details/39002086346070.med.yale.edu