I. Introduction
Is there evidence for the existence of gender differences in newborn behavior? While there is little research on human newborns, animal research has demonstrated sexual differentiation of the gonads in male and female fetuses; male fetuses have higher levels of gonadal hormones than females (Haseltine, & Ohno, 1981). These hormonal differences are also important in determining brain sexual dimorphism. High levels of hormones among male fetuses affect the central nervous system, giving rise to the masculinization of the encephalon (Kelly, Ostrowski, & Wilson, 1999; LeVay, 1993).

There are also physical differences between boys and girls; boys are bigger than girls regarding weight and length (Thomas, Peabody, Turnier, & Clark, 2000). In this sense, there are reports that support the notion that male newborns are more vulnerable than females; they have higher metabolic demand (Stevenson, Verter, & Fanaroff, 2000). Furthermore, sensory development research of full-term, typically developing newborns shows differences in hearing between the sexes with girls showing higher auditory sensitivity than boys (Cassidy & Ditty, 2001). In relation to behavioral differences, Lundqvist and Sabel (2000) found that girls scored significantly higher than their counterparts in 4 of the 7 items of the social-interactive NBAS- cluster.

Using the literature as a framework, we designed a study to examine gender differences in neonatal behavior. We hypothesized that girls would demonstrate superior functioning in their social-interactive capacities than boys, as assessed by the Neonatal Behavioral Assessment Scale (NBAS).

II. Method
A sample of 188 healthy full-term newborns (88 boys and 100 girls) were examined when they were between 48 and 80 hours old. All infants were evaluated with the Spanish version of the NBAS, 3rd. edition by trained clinicians. Clinicians were blind to the objectives of the study.

Data analyses were conducted using SPSS (Statistical Package for Social Sciences, version 12.0). Mean scores were calculated for each behavioral item of the NBAS and then compared using between-group, t-test analyses between males and females.

III. Results
Comparison tests showed significant gender differences on 5 of the 34 items of the NBAS (Animate Auditory Orientation, Alertness, Quality of Alertness, State Regulation, Irritability). A box plot illustration (see Figure 1) presents the median value scores on each of the 4 items for girls and boys, representing that girls scored significantly higher than boys on the following items: Animate Auditory Orientation, Alertness, Quality of Alertness and State Regulation whereas, boys showed scored higher on Irritability.