We found similar motivational effort for viewing normal babies in both groups despite lower attractiveness ratings by the men. On the other hand, women rated abnormal babies' faces as unattractive as did men, but they expended more absolute effort to decrease the viewing times of these same faces. This group difference was not explained by the overall level of key-press activity and by gender of the baby face. The small number of participants renders our results preliminary however, pending replication in follow up studies.
The performance of work in order to continue viewing pictures of healthy babies is consistent with preclinical studies where rat pups served as a reinforcing stimulus in bar-pressing operant chamber procedures 
. This preference of laboratory animals was, however, narrowly restricted to mothers i.e., nulliparas avoided while postpartum animals were attracted to the pups 
. In humans, on the other hand, images of unfamiliar babies appear to be reinforcing in general, i.e., regardless of the gender and/or of parental status of the participants, as parents and non-parents of both genders activated brain motivational regions when exposed to such stimuli 
The present data extend our prior findings of increased effort to rating ratio exhibited by men with regard to attractive female faces, which we referred to as a gender-specific incentive sensitization 
. Although there were methodological similarities between the latter 
and current study (e.g., enrollment of healthy men and women as well as the use of an analogous key press/rating procedure), there were also important differences, including a novel pictorial stimulus and a decreased valuational assessment rather than an increased motivational effort displayed by men. Together, our current and previous results suggest that, in comparison to women, men may be more motivationally sensitized to procreation-related esthetic stimuli. An alternative explanation is that higher attractiveness ratings for normal babies could reflect societal acceptability demands still perceiving women as predominant caregivers for the young 
. Resolution of the motivational vs. social origin interpretation of observed gender differences will require additional studies utilizing various types of esthetic stimuli.
Our results generalize motivational sensitization processes to women and suggest a different mechanism by which these processes may be mediated. Thus, in some domains, women may be predominately driven by negative reinforcement and/or avoidance leaning rather than by positive reinforcement mechanisms that may be more typical of men. This assumption may provide at least partial explanation for excessive reactivity to stress and other negative stimuli in women 
Studies of abandoned and neglected children firmly link their abnormal appearance to the maltreatment by the caregivers 
. This may be to some extent because adults' are unconsciously motivated to care for infants with healthy facial features indicating fitness for survival and to exclude the least fit 
. The abandonment and neglect data 
along with our findings may thus challenge the commonly held view of unconditional maternal love and acceptance of the offspring 
. If mother's love is not unconditional, what is the condition? The present results provide indirect support for Weiss' 
idea that babies' esthetic appearance has a motivating influence on the adults' caretaking behavior. Clinical implications of our findings in terms of predicting potential for abuse and neglect of children with abnormal facial characteristics may transpire in cross-sectional and prospective clinical trials involving populations at risk. Further research is also needed to determine gender differences in the neural substrate underlying incentive sensitization processes and how it may be involved in psychopathologies characterized by gender-specific courses, such as schizophrenia, substance use disorders and major depression.