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Physiology and Maturation
Puberty is the transitory stage in human development in which a person goes from a child into a reproductively mature adult, in other words, puberty is the process of sexual maturation in humans. The onset of puberty varies between boys and girls, with boys usually starting around 11-12 years of age and 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 determine due to menarche. While there is variation in the onset time and magnitude, the sequence of events stays more or less consistent, variations in the sequence can indication a pathological condition.

Dietary influence
Differences in quality and quantity of nutrition account for one of the strongest environmental factors that alter the onset of puberty. Evidence has linked childhood obesity in girls with early pubertal timing, referencing an increased amount of body fat as a signal for the brain to initiate puberty and due to an excess of available energetic resources, since developing a fetus is very energetically demanding.

Illness
Disease and chronic illness in childhood can lead to a delay in pubertal timing in boys and girls. Inflammatory diseases, parasitic infections, and other illnesses that affect nutritional intake, specially chronic ones, are energetically costly and energy and resources has to be allocated into maintenance and health, sometimes taking energy from growth or reproduction, stunting or delaying them.

Genetics and environmental causes
Variation in pubertal timing has been directly found to be due through direct genetic association between mothers and daughters in 46% of the population studied. It is believed that an androgen receptor gene, but the specific gene has not been found. Chemicals and hormones found in the environment and plastics such as Bisphenol A (BPA) have been thought to affect sexual development in humans at the prenatal or postnatal stage. According to the Centers for Disease Control and Prevention (CDC), BPA found in plastic bottles and containers leaches into foods and liquids when warmed up, as in the case of plastic baby bottles, and traces of the chemical were found in more than 90% of the U.S. population studied. BPA is of concern because it interferes with the actions of estrogen which is needed as a developmental and reproductive regulator.

Stress and psychosocial factors
Most of the studies have reported that menarche may occur a few months earlier in girls in high-stress households, whose fathers are absent during their early childhood, who have a stepfather in the home, who are subjected to prolonged sexual abuse in childhood, or who are adopted from a developing country at a young age. Conversely, menarche may be slightly later when a girl grows up in a large family with a biological father present. However, when the stress is severely high and potentially life threatening such as in times of war, the onset of puberty has been delayed.