I've had several different diagnoses over the years: ADD, depression, OCD, bipolar, BPD ... the current theory is Asperger's. I don't fit real neatly into any one category but I certainly seem to have *something* and I do take meds to manage it, whatever it is.
Brother-Husband takes meds too and has occasional deep depressions that come and go, but most of the time he seems quite "normal." Snowbunny does a fair bit of internalizing but her meds are just for her blood pressure.
One possible theory for this thread is that maybe people who don't "fit in" will have more of a tendency to try off-the-beaten-path things such as polyamory. And any mental disorder is likely to make it hard for a person to "fit in."
But, that's all speculation.
Love means never having to say, "Put down that meat cleaver!"