Final answer:
Alfred Kinsey proposed the concept of sexuality as a continuum and introduced the Kinsey scale for sexual identity development. His work, especially in 'Sexual Behavior in the Human Male', marked a significant departure from the binary view of sexuality and laid the groundwork for understanding sexual orientation as a spectrum.
Step-by-step explanation:
The individual who proposed a series of stages for sexual identity development was the notable sex researcher Alfred Kinsey. Alfred Kinsey was a groundbreaking figure in the study of human sexuality. In particular, he conceptualized sexuality as a spectrum rather than a binary of gay or straight. Kinsey introduced the Kinsey scale, a six-point rating scale that ranges from exclusively heterosexual to exclusively homosexual, which postulated that most individuals fall somewhere in between, challenging the strict dichotomies that were commonly accepted at the time. Kinsey's views were expressed in his seminal 1948 work, Sexual Behavior in the Human Male.
While Sigmund Freud introduced the stages of psychosexual development, which highlighted different erogenous zones during the developmental stages of childhood, and Erik Erikson expanded upon Freud's ideas to include social aspects and a lifespan perspective, it was Kinsey who notably diverged to discuss sexual orientation development in a continuous and fluid manner. Dr. John Money also contributed to the field with his theory of psychosexual neutrality, asserting that gender identity is largely shaped by social interactions after birth.