Some experts distinguish sex characteristics at three different levels: Primary (e.g., reproduction, ova/sperm production). Secondary (e.g., anatomical size, bone structure, body hair distribution). Tertiary (e.g., posture, facial expression, leg position). (Eakins, Barbara Westbrook, and R. Gene Eakins. Sex Differences in Human Communication. p 4. Boston:Houghton Mifflin Company, 1978.)