Introduction

Have you ever pondered the complex interplay of factors that not only determine our sex but also shape how we behave and interact with the world? In this chapter, we will delve into the captivating world of sexual behavior and development, unraveling the intricate biological, psychological, and environmental threads that weave the rich tapestry of sex differences in both humans and other species.

Before outlining the structure of this chapter, I'd like to clarify some essential distinctions. First, we need to differentiate between 'sex' and 'gender.' While 'sex' refers to the biological differences between females and males, including chromosomes, hormone profiles, internal and external sex organs, 'gender' encompasses the socially and culturally constructed roles, behaviors, expressions, and identities of girls, women, boys, men, and gender-diverse people. Secondly, it's important to note that although this chapter occasionally uses the terms “males” and “females” to refer to the biological categories typically associated with female (XX chromosomes in mammals) or male (XY chromosomes in mammals), we must acknowledge the considerable variation in how these biological attributes are expressed, contributing to a broad spectrum of biological sex characteristics. Now, let's begin our exploration of how this chapter will unfold.

We begin by exploring the fundamentals of sexual reproduction and sexual dimorphism. Why does sexual reproduction exist, and what evolutionary advantages does it offer species that reproduce this way? You'll discover how the differentiation between sexes has evolved over millions of years, leading to a wide spectrum of physical and behavioral traits that define male and female forms across animal species. We will also examine sexual dimorphism beyond mere physical differences, focusing on how these distinctions manifest in behavior and even influence non-sexual traits.

As we delve deeper, the mechanisms that determine and differentiate sex will unfold. How are these mechanisms influenced by genetics and the environment? What roles do hormones play from the moment of conception and throughout life? Here, you'll learn about the organizational and activational effects of hormones, understanding how they sculpt our bodies and minds from the womb onwards and continue to influence us through adulthood.

The chapter progresses to explore how these foundational concepts of sex and development impact brain function and behavior. We will discuss how genetic, hormonal, and environmental factors converge to mold sex-specific brain architectures, leading to distinct patterns in stress responses, motivation, and social behaviors. This section will also highlight the critical role of epigenetic mechanisms, which illustrate how our environments interact with our genetic blueprint to produce lasting effects on our behavior and mental health.

Lastly, we will touch on the vulnerability to psychiatric diseases, examining how sex differences in brain and behavior can influence susceptibility to conditions like anxiety, depression, and other mental health disorders. You will see how new research is further uncovering the roles of brain-resident immune cells like microglia and mast cells in these processes, offering new perspectives on why males and females might experience and respond to psychological stress differently.

Throughout this chapter, our discussion will be grounded in scientific research, while also considering the broader implications of these findings for understanding human diversity and improving health outcomes. By the end of this chapter, you will have a comprehensive understanding of the intricate biological and social constructs that influence sexual behavior and development, equipped with a deeper appreciation for the complexity of life and the nuanced ways in which sex and gender manifest across the spectrum of life.