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[1]: 24 [4] Sex differences in the brain have been found in many structures, most notably the hypothalamus and the amygdala. [2] However, few of these have been related to behavioral sex differences, and scientists are still working to establish firm links between early hormones, brain development and behavior. [2]
The sex of the fetus may be discerned by ultrasound as early as 11 weeks' gestation. The accuracy is relatively imprecise when attempted early. [15] [16] [17] After 13 weeks' gestation, a high accuracy of between 99% and 100% is possible if the fetus does not display intersex external characteristics. [18]
At an early stage in embryonic development, both sexes possess equivalent internal structures. These are the mesonephric ducts and paramesonephric ducts . The presence of the SRY gene on the Y chromosome causes the development of the testes in males, and the subsequent release of hormones which cause the paramesonephric ducts to regress.
Sexual differentiation is the process of development of the sex differences between males and females from an undifferentiated zygote. [1] [2] Sex determination is often distinct from sex differentiation; sex determination is the designation for the development stage towards either male or female, while sex differentiation is the pathway towards the development of the phenotype.
Studies testing for differences in behavior (i.e. temperament) tend to yield inconsistent results, [19] while those testing perception and cognition are typically more consistent. [5] Though supporting evidence exists, whether or not prenatal testosterone transfer occurs in humans remains debatable.
In girls, gender nonconformity comprises dressing like and playing with boys, showing interest in competitive sports and rough play, lacking interest in conventionally female toys such as dolls and makeup, and desiring to be a boy". This gender nonconformist behavior typically emerges at preschool age, although is often evident as early as age 2.
Also, most low-frequency sounds (less than 300 Hz) can reach the fetal inner ear in the womb of mammals. [20] Those low-frequency sounds include pitch, rhythm, and phonetic information related to language. [21] Studies have indicated that fetuses react to and recognize differences between sounds. [22]
The results found by the researches support the notion that gender differences in spatial abilities arose during human evolution such that both sexes cognitively and neurologically developed to behave adaptively. However, the effect of socialization and environment on the difference in spatial ability is still open for debate. [13]