Why was Hannah Ocuish believed to have been mentally disabled?

Trying to apply modern medical or psychological terminology to past people or events ranges from difficult to nearly impossible. The historical process only allows us to look at what was written, and in the case of an individual accused of the murder of a six-year old child that writing will be strongly biased. Useful conclusions drawn from this type of evidence unfortunately are also usually biased by what the reader wants to believe, by the readers (and writers) modern perspectives.

The first source I find indicating a possible mental disability for the accused Hannah Ocuish is in the 1987 book Death penalty for juveniles, by Victor L. Strieb. (This book can be borrowed and read at archive.org)

Strieb discusses the Ocuish case primarily on pgs 74-75, where he states (emphasis mine)

The mothers alcoholism and other problems caused her to abandon Ocuish and two older brothers in their early childhood. The Ocuish children became notorious for robbing and harassing local citizens. Ocuish was placed with a New London family by the local selectmen but her mother was unsatisfied with that arrangement, took Ocuish away when she was six years old, and placed her with a widow who lived just outside of New London. Apparently retarded, the pre-pubescent Ocuish was widely known as a 'fierce young savage." (7)

This section is footnoted (7) to the source F.Caulkins, which is from pg 576 of The History of New London Connecticut,published in 1852 by Frances Manwaring Caulkins.

The relevant entry from Caulkins read like this (all emphasis mine)

On the 20th of December 1786 Hannah Occuish was executed in New London for the willful murder of Eunice daughter of James Bolles. The crime was committed July 21st 1786 The perpetrator was an Indian girl of Pequot parentage only twelve years and nine months old, her victim was six years and six months old. The murdered child was found about ten o clock in the morning on the Norwich road two or three miles from town. She lay under the wall from which heavy stones had been thrown down upon her body. On examination it was discovered that her death could not have been the result of accident and after a day or two suspicion having rested on Hannah Occuish who lived with a widow woman near by she was examined and confessed the crime. It was a case of cruel and malicious murder growing out of a dispute that occurred in a strawberry field some days before. The fierce young savage nursing her wrath and watching for an opportunity to take revenge at length came upon her victim on her way to school alone and after coaxing and alluring her into a wood fell upon her and beat her to death. The only alleviating circumstances in this case were the extreme ignorance and youth of the criminal. These were forcible arguments but not at that day of sufficient weight to reprieve from execution. The gallows was erected in the rear of the old meeting house near the corner of Granite Street. The sermon on the occasion was delivered by the Rev Henry Channing from Yale College who was then preaching as a candidate to the First Congregational Society.

It is clear that the citation by Strieb references the 'fierce young savage' phrase by Caulkins, but there is also an indication of a possible mental disability in the line concerning 'extreme ignorance'. (Caulkins 19th century bias towards Native Americans, and also may have a personal bias as the Caulkins families an Bolles families have intermarriages.)

Another source mentioned by Strieb is the genealogical work The Genealogy of the Bolles family in America, by John A. Bolles. This work is basically redundant as cites Caulkins directly.

Caulkins does lead us to what could be considered a primary source however, the sermon by Henry Channing, published under the weighty title

God admonishing his people of their duty, as parents and masters. A sermon, preached at New-London, December 20th, 1786. Occasioned by the execution of Hannah Ocuish, a mulatto girl, aged 12 years and 9 months. For the murder of Eunice Bolles, aged 6 years and 6 months. / By Henry Channing, M.A.

This can be read at the umich site. Of particular interest is the appendix section, which provides many of the details carried forward in one form or another by later sources. One section contains some lines which may raise some concerns over the mental state of the girl:

When she was brought to the bar to receive sentence of death, her stu|pidity and unconcern astonished every one. While that benevolent ten|derness which distinguishes his honor the Chief Justice, almost prevented utterance, and the spectators could not refrain from tears; the prisoner alone appeared scarcely to attend.About a fortnight before her execution she appeared to realize her dan|ger, and was more concerned for herself. She continued nearly in the same state until the Monday night before her execution: when she appeared greatly affected; saying, that she was distressed for her soul. She conti|nued in tears most of Tuesday, and Wednesday which was the day of exe|cution. At the place of execution she said very little—appeared greatly afraid, and seemed to want somebody to help her.—After a prayer a|dapted to her unhappy situation, was offered to HEAVEN, she thanked the sheriff for his kindness to her, and then passed into that state which never ends.

Whether or not these lines indicate a diminished capacity or just the fact that the child was unable to comprehend the gravity of the situation are questioned by Streib in his book (pg 157), in reference to Ocuish and other cases of juvenile execution he discusses:

All seven of these young persons showed a lack of understanding and concern about the death penalty before committing their crimes or their impending executions after being condemned. For the younger ones and the more retarded ones, that may simply have have stemmed from ignorance and immaturity. For the older ones, it may be illustrative of the adolescents' lack of understanding of the nature of death.

So it would appear that later sources claiming mental disability may be building their cases on the earlier mentions of 'extreme ignorance' and 'her stupidity and unconcern'. If these mentions are actual indications of mental illness or diminished capacity we can't know with any certainty.