The methods that female serial killers use for murder are frequently covert or low-profile, such as murder by poison (the preferred choice for killing). Other methods used by female serial killers include shootings (used by 20%), suffocation (16%), stabbing (11%), and drowning (5%). They commit killings in specific places, such as their home or a health-care facility, or at different locations within the same city or state. A notable exception to the typical characteristics of female serial killers is Aileen Wuornos, who killed outdoors instead of at home, used a gun instead of poison, and killed strangers instead of friends or family. One "analysis of 86 female serial killers from the United States found that the victims tended to be spouses, children or the elderly". Other studies indicate that since 1975, increasingly strangers are marginally the most preferred victim of female serial killers, or that only 26% of female serial killers kill for material gain only. Sources state that each killer will have her own proclivities, needs and triggers. A review of the published literature on female serial murder stated that "sexual or sadistic motives are believed to be extremely rare in female serial murderers, and psychopathic traits and histories of childhood abuse have been consistently reported in these women."