In a review of published literature on female serial murder, the most common motive identified was material gain. [i] Sexual or sadistic motives are believed to be extremely rare in female serial murderers. Psychopathic traits and histories of childhood abuse have been consistently reported in these women. In a study of 105 female serial killers, the preferred method of killing was poisoning. [ii] An analysis of 86 female serial killers from the U.S. found that the victims tended to be spouses, children or the elderly. [iii] Sometimes referred to as "black widow" killers, these women tend to be geographically stable and live in the same area where their offenses occurred.
But you bring up an interesting point -- could a female black widow type serial killer keep "trophies" or some type of memento as a method of reliving the power and control she had over her victims -- literally holding their lives in her hands? I have not yet heard of such a case, but this is an interesting hypothesis.
[i] Frei A., Vollm B., Graf M., Volker D. Female serial killing: Review and case report. Criminal Behavior and Mental Health 2006 16: 167-176.
[ii] Wilson W., Hilton T. Modus operandi of female serial killers. Psychological Reports 1998 82: 495-498.
[iii] Kelleher M. Kelleher C. Murder Most Rare: The Female Serial Killer. Praeger: Westport, CT, 1998.
RITA, Melbourne, Fla.: Were mother and daughter evaluated by a psychiatrist? It was never mentioned in the show. Was Stacey tested for multiple personalities? I ask because Stacey's mother and friends seem unflappable in their support. However, her flat affect was incredible, especially when the prosecutor was yelling. Was she on drugs during the trial? KNOLL: To my knowledge, neither mother nor daughter were psychiatrically evaluated. For Castor to have been psychiatrically evaluated for the purposes of the trial, she would have had to open this door herself by raising some type of mental health defense, which she did not. She simply maintained her innocence.
I do not know whether Castor was on medications during the trial, as this information would have been confidential, unless she had raised a mental health defense. I too was struck by her lack of emotional expression. Please see one of my previous answers re: this issue and what factors can potentially cause a defendant to appear emotionless besides simple lack of remorse.
COLLEEN, Rochester, N.Y.: I don't understand the difference between psychopathy and mental illness. Aren't all serial killers mentally ill?
KNOLL: Most data on psychiatric diagnoses of serial murderers comes from individual case studies and retrospective analyses. The majority of these studies have suggested a common constellation of diagnoses: psychopathy, antisocial personality, sexual sadism and other paraphilias (sexual deviancies). More recent and well designed comparison studies have yielded similar findings. For example, a study comparing sexual murderers to other general sex offenders found that the sexual murderers had greater levels of psychopathy, sadism, fetishism, and transvestism. [i] There have been a few case reports of of serial killers who have been psychotic (lost touch with reality, have delusions, etc.), but these are rare.