Subsequent studies have agreed on a commonality of motives in cases of maternal filicide. These motives are: (1) the mother's mental illness, often seen as "pathological," "acutely psychotic," or "mentally ill" killings, (2) lack of bonding with the child, manifested as "neonaticide," "unwanted child," or "ignored pregnancy" deaths, and (3) inadequate parenting, resulting in "accidental," "discipline-related," or "neglect" deaths.
Recent studies focus on more than just the motive, but on the nature of the mother-child relationship. Forensic psychiatric evaluations of women criminally charged with the deaths of their children found the following characterizations of the mother-child relationship: abusive / neglectful mothers, psychotic / depressed mothers, retaliatory mothers, psychopathic mothers, and detached mothers.
Abusive, Psychopathic, or Retaliatory mothers
Abusive / neglectful mothers are unable to set normal behavioral bounds with their child, vacillating from excessive discipline to no discipline. Retaliatory mothers are similar to Resnick's spousal revenge category. The psychopathic mother has an insensitive relationship with their children, using the child to fulfill their own needs.
The detached mother
The detached mother category reflects mothers that have not developed a bond with their child during pregnancy. Researchers talk about the "massive denial" of these women who kill their child. Typically, these mothers deny their pregnancy, often to the point where physical symptoms do not manifest until the actual "surprise" birth. The detached mother may deny the pregnancy out of resentment of the child, a lack of communication within her social network, or a fear of rejection by her family or friends. Interestingly, the families and support systems of these women do not notice the changes in the young woman.
The mother may actually have a dissociative event during childbirth, not remembering the birth or even killing the child at birth.  Neonaticide, the killing of a child in the first day of life, may occur if these women give birth in an isolated area or alone. These women were later horrified to find out what they had done.
The psychotic / depressed mother
The psychotic / depressed mother perceives her child through the lens of her particular illness. The illness may be a previously diagnosed clinical disorder, such as schizophrenia, depression, substance abuse, or bipolar disorder. The mother may be suffering from a personality disorder, defined as "an enduring pattern of inner experience and behavior that deviates markedly from the expectations of an individual's culture, is pervasive and inflexible, has an onset in adolescence or early adulthood, is stable over time, and lead to distress or impairment."  Three personality disorders relevant to filicidal mothers are dependent, antisocial, and borderline personality disorder. Impulsive actions, poor judgment, and instability in personal relationships and self-image characterized borderline personality disorder.
Just as the families did not notice the pregnancy of denial mothers, the families of young women suffering from clinical or personality disorders often ignore "the elephant in the living room" and deny the problem.  The result is that these young women are often at risk because they do not get the diagnosis, treatment, and help that they need due to unacknowledged or unrecognized mental disorders.
Mothers with Bipolar Disorder
One such clinical disorder that may go undiagnosed is bipolar disorder. Bipolar disorder usually begins between ages 15 and 30, and some forms are more common in women.  Symptoms of bipolar disorder, also known as manic depression, are extreme mood swings ranging from manic highs to intense lows.  Because these mood swings are on a continuum, and may even not appear at times, some people may go undiagnosed because they don't seek treatment, their condition is mistaken for depression, or because their symptoms don't meet current diagnostic criteria.  Complicating the picture, bipolar disorder frequently is accompanied by other disorders. 
The mood swings may last for weeks, months, or even years.  In the manic phase of bipolar disorder, the signs and symptoms include: extreme optimism, inflated self-esteem, poor judgment, agitation, risky behavior, spending sprees, increased sexual drive, decreased need for sleep, and a tendency to be easily distracted.  During the depressive phase, it is easy to see how signs and symptoms of bipolar disorder may masquerade as depression: sadness, hopelessness, suicidal thoughts or behavior, anxiety, guilt, sleep problems, appetite problems, fatigue, loss of interest in daily activities, problems concentrating, irritability, and chronic pain without a known cause. 
Continued, in part 3 ....
This four-part series of posts "Filicide is Different" continues next Friday, as part of Friday's In Depth Look / Friday's Legal Memo. The third part of the series discusses progressive postpartum depression and the Andrea Yates case.
 Geoffrey R. McKee, Why Mothers Kill, A Forensic Psychologist's Casebook 28 (2006).
 Oberman, supra, at 53.
 Oberman, supra, at 71.
 Arlene M. Huysman, A Mother's Tears 52 (1998).
 Mayo Clinic, Bipolar Disorder, at 4, available at http://www.mayoclinic.com/health/bipolar- disorder/DS00356/DSECTION=symptoms.
 Id., at 1
 Id., at 4.
 K. Cauldwell, Living With Bipolar Disorder: One Woman's Journey Through Diagnosis, Understanding, and Acceptance, at 2 http://www.associatedcontent.com/article/106100/living_with_bipolar_disorder_one_womans.html?cat=5.
 Mayo Clinic, supra, at 2.
 Mayo Clinic, supra, at 2-3.