ending by 16-17, and girls starting around 10-11 and ending at 15-17. Activity in the hypothalamic–pituitary–gonadal axis (HPG axis) initiates puberty by secreting gonadotropin-releasing hormone (GnRH) from the hypothalamus into the anterior pituitary. The anterior pituitary releases the gonadotropins luteunizing hormone (LH) into the ovaries, which produce estrogen, and follicle-stimulating hormone (FSH) into the testes, which produce testosterone. The central event in puberty for females is menarche, the first menstrual bleeding. For males, it is the first ejaculation. The onset of menarche is easier to determine due to the evidence of menstrual bleeding, while the first ejaculation for males is usually self reported. In evolutionary context, it is assumed that human physiology has been modeled through natural selection to maximize reproductive success by allotting energy and resources through trade-offs. This period of reproductive maturation sees the onset of primary sexual characteristics, the production of gametes and hormones by the gonads, and secondary sexual characteristics. Secondary sexual characteristics include adolescent growth spurt, pubic and axillary hair, genital enlargement, breast development in girls, beard growth in boys, increase in subcutaneous fat, increase in muscle mass, and widening of the pelvis in girls. While there is variation among individuals, secondary sexual characteristics tend to develop in a sequence. For girls, breast development is followed by the appearance of pubic hair, followed by menarche, and fat deposition and broadening of the hips occurring as the completion of breast development approaches. For boys, enlargement of the penis and testicles occurs, followed by pubic and axillary hair growth, voice change, facial hair growth, and muscle mass increase. This period is also a time of cognitive and psychosocial development where social relationships, skills, and experiences outside of the core family are explored. Pubertal variation While puberty is a consistent progression of events culminating in reproductive maturity, there is wide variation in age of onset of puberty and the magnitude of the changes that can be caused by a variety of different influences. Since the mid 19th century the global age of menarche has significantly decreased. Dietary composition, disease, psyschosocial circumstances, developmental conditions, genetics and epigenetics, and other environmental factors can all affect the age of the onset of puberty. These factors can come together and in terms of evolutionary trade offs, alter the allocation of energy into growth, maintenance, or reproduction, as best needed for survival. Most research focuses on female puberty because it is easier to