last updated:

April 9, 2021

[Brain Image]    

PSY 340 Brain and Behavior

Class 23: Sex and Hormones



March 17, 2005: The human X chromosome has been fully decoded and the results published in the journal, Nature. The "X" chromosome, one of the two sex chromosomes, is a DNA sequence of ca. 1.5 million base-pairs  ca. 154 million base-pairs coding for ca. 800 proteins in every cell in the human body (vs. ca. 59 million base-pairs in the "Y" chromosome coding for ca. 70 proteins). The sexes are differentiated by the presence of two X chromosomes in females and an X and Y chromosome in males.

How are women and men different over and above the obvious physical differences?

Are there real differences between the sexes? If so, why?

            Differentiation]A. Becoming male or female (that is, sexual differentiation)

Development in males
Development in females

Actions of
            HormonesHormones: Androgens & Estrogens


B. Organizing Effects of Sex Hormones

Organizing Effects -> Causes permanent structural differences to develop gradually in the body. Usually happens during "sensitive" periods in development of the fetus and newborn infant. However, sex hormones also have organizing effects at puberty, e.g. development of female breasts or male facial hair.

Activating Effects -> Causes temporary behavior to occur over seconds, minutes, hours, or days following exposure to hormone(s). Some such effects can be emotional arousal during pregnancy, aggressive behaviors, heightened sexual receptivity, etc.

We have already seen the organizing effect of sex hormones above in the initial development of the sexual organs. What about the continuing organizing effects during pregnancy?

   1. Sex Differences in the Hypothalamus

        POA AH

   2. Sex Differences in Childhood Behavior

C. Activating Effects of Sex Hormones in Humans

1. Sexual Behavior in Humans

In males

[menstrual cycle]In females: the menstrual cycle

  Interactive effects of hypothalamus and pituitary gland

Sex Hormones & Nonsexual Characteristics

Male-Femail Brain Connectivity

D. Parental Behavior

Rat Pups[Pups]1. Behavior in Mammals

2. Human Parental Caregiving (this is not in text but required)


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This page was first posted March 20, 2005.