According to Penn State University professor David Puts, men have deeper voices than women because boys experience a rise in testosterone during puberty that makes their vocal cords grow longer and thicker, causing their voices to drop as much as one full octave. Girls experience vocal cord changes during puberty as well, but their voices only deepen slightly compared to boys.
According to KidsHealth, boys and girls grow a larger and thicker larynx during puberty. Before puberty, the larynx is small and the vocal cords are thin. This change is much more significant in boys than in girls. A boy's larynx begins to grow so large that it starts to protrude from the front of his throat. The protruding larynx is often referred to as an "Adam's apple." Until the larynx stops growing, a boy's voice may "crack" frequently. Once the larynx finally stops growing, a boy's voice becomes much deeper.
A girl's larynx only grows slightly during puberty, notes KidsHealth. The change is barely noticeable, which is why girls do not have an "Adam's apple." A girl's voice only deepens a couple of tones during this change, and girls do not experience "cracking." The growth of the larynx is less profound in girls than boys because girls have lower testosterone levels.