Yes, the Fletcher's lawyers might argue Lacey's parents both suffered from diminished responsibility:
"diminished responsibility (or
diminished capacity) is a potential
defense by
excuse by which
defendants argue that although they broke the
law, they should not be held fully
criminally liable for doing so, as their mental functions were "diminished" or impaired." For example, after years of being primary caregiver's to Lacey, her parents became numb to the residual aspects of their daughter's failing health and well-being. They didn't understand the implications of their actions over a very long period of time, and how their daughter was becoming more and more incapacitated even though purid sanitation was out of control in their living quarters and their daughter was immersed in waste and infested with parasites. What sane person would live in a home like this??? How successfully their defense will be is anyone's guess. "The effect of the defense varies between the
jurisdictions. In some, it will result in full excuse and therefore produce a verdict of "
not guilty". In others, it offers only exculpation to a degree, resulting in the substitution of a lesser offence (e.g.,
manslaughter instead of murder) or a mitigated
sentence."
en.wikipedia.org
I can't image how else their attorney will frame Lacey's parents defense, but there seems to be some corroborating evidence that the Fletchers were considered good citizens and good parents.