Sex differences and the role of sex hormones in face development and face processing

Sex differences have been identified in both external appearance of faces (e.g. Bulygina et al., 2006; Weston et al., 2007) and the way information about faces is extracted by our brains, that is in face processing (e.g. Tahmasebi et al., 2012; Hampson et al., 2006). The mechanisms leading to the development of such sex differences are not well understood. This thesis explores the role of sex hormones in face development and face processing. Data from two large-scale studies (Saguenay Youth Study and Imagen, with n=1,000 and 2,000, respectively) and four smaller datasets (Cycle-Pill Study, n=20; Pill Study, n=20; First Impression Study, n=120, and Twin Study, n=119) were used to explore the effects of sex and sex hormones on face development (head MR images, MRI-face reconstruction) and face processing (functional MRI data, eye-tracking data). Shape of male and female faces was influenced by both prenatal and pubertal androgens. Facial signature of prenatal androgens, identified by the sex-discordant twin design, was found also in an independent dataset of female adolescents (singletons) and we showed that prenatal androgens, indexed indirectly by the facial signature, were associated with larger brain size. We propose that this facial signature might be used, similarly to digit ratio, as an indirect index of prenatal androgens. Variability in postnatal sex hormones due to the use of oral contraception and the phase of menstrual cycle influenced brain response to faces. Using the same dynamic face stimuli as in the functional magnetic resonance imaging (fMRI) study, we showed that eye-movements scanning the face did not differ between the users and non-users of oral contraception. We conclude that effects of sex hormones can be observed in both the face and the brain and that these effects help us understand sex differences in face shape and face processing. **This version does not contain the previously published journal articles reproduced in the printed thesis (appendices 1-3). For details see p. 188. *

Ovulatory shift, hormonal changes, and no effects on incentivized decision-making

Employing an incentivized controlled lab experiment, we investigate the effects of ovulatory shift on salient behavioral outcomes related to (i) risk preferences, (ii) rule violation, and (iii) exploratory attitude. As evolutionary psychology suggests, these outcomes may play an important role in economic decision-making and represent behavioral aspects that may systematically vary over the menstrual cycle to increase the reproductive success. Exploiting a within-subjects design, 124 naturally cycling females participated in experimental sessions during their ovulation and menstruation, the phases between which the difference in the investigated behavior should be the largest. In each session, hormonal samples for cortisol, estradiol, and testosterone were collected. The group of women was also contrasted against an auxiliary reference group composed of 47 males, who are not subject to hormonal variations of this nature. Our results reveal no systematic behavioral differences between the ovulation and menstruation phases

Sex differences and the role of sex hormones in face development and face processing

Sex differences have been identified in both external appearance of faces (e.g. Bulygina et al., 2006; Weston et al., 2007) and the way information about faces is extracted by our brains, that is in face processing (e.g. Tahmasebi et al., 2012; Hampson et al., 2006). The mechanisms leading to the development of such sex differences are not well understood. This thesis explores the role of sex hormones in face development and face processing. Data from two large-scale studies (Saguenay Youth Study and Imagen, with n=1,000 and 2,000, respectively) and four smaller datasets (Cycle-Pill Study, n=20; Pill Study, n=20; First Impression Study, n=120, and Twin Study, n=119) were used to explore the effects of sex and sex hormones on face development (head MR images, MRI-face reconstruction) and face processing (functional MRI data, eye-tracking data). Shape of male and female faces was influenced by both prenatal and pubertal androgens. Facial signature of prenatal androgens, identified by the sex-discordant twin design, was found also in an independent dataset of female adolescents (singletons) and we showed that prenatal androgens, indexed indirectly by the facial signature, were associated with larger brain size. We propose that this facial signature might be used, similarly to digit ratio, as an indirect index of prenatal androgens. Variability in postnatal sex hormones due to the use of oral contraception and the phase of menstrual cycle influenced brain response to faces. Using the same dynamic face stimuli as in the functional magnetic resonance imaging (fMRI) study, we showed that eye-movements scanning the face did not differ between the users and non-users of oral contraception. We conclude that effects of sex hormones can be observed in both the face and the brain and that these effects help us understand sex differences in face shape and face processing. **This version does not contain the previously published journal articles reproduced in the printed thesis (appendices 1-3). For details see p. 188. **EThOS - Electronic Theses Online ServiceGBUnited Kingdo