Final answer:
The pelvic girdle is most effective in determining the sex of a skeleton due to the differences in shape between male and female pelvises, which reflect adaptations for childbirth in females.
Step-by-step explanation:
The pelvic girdle is the most effective set of bones in determining the sex of a skeleton. The female pelvis is generally broader, shallower, and has a wider angle between the pubic bones when compared to the male pelvis, which is typically narrower, deeper, and has a more acute pubic angle. These differences are a result of adaptations for childbirth in females. Specifically, the bones that are most informative are the ilium, ischium, and pubis, which fuse together to form the hip bones in adults. The broad differences between male and female pelvic bones are part of the phenomenon known as sexual dimorphism, which is the variation in the appearance between males and females of the same species.