in 2500 have a 45,X one. Girls have a female reproductive system. Some intersex children with ambiguous genitals, as well as transgender children may also be classified or self-identify as girls. Girls' bodies undergo gradual changes during puberty. Puberty is the process of physical changes by which a child's body matures into an adult body capable of sexual reproduction to enable fertilization. It is initiated by hormonal signals from the brain to the gonads. In response to the signals, the gonads produce hormones that stimulate libido and the growth, function, and transformation of the brain, bones, muscle, blood, skin, hair, breasts, and sexual organs. Physical growth—height and weight—accelerates in the first half of puberty and is completed when the child has developed an adult body. Until the maturation of their reproductive capabilities, the pre-pubertal, physical differences between boys and girls are the genitalia. Puberty is a process that usually takes place between 10 and 16 years, but these ages differ from person to person. The major landmark of female puberty is menarche, the onset of menstruation, which occurs on average between 12 and 13. According to a 2010 Canadian study, the variation of age in which menstruation begins had a "statistically significant" relation to where the child was living, household income, and family type. Gender and environment Biological sex interacts with environment in ways not fully understood. Identical twin girls separated at birth and reunited decades later have shown both startling similarities and differences. In 2005, professor Kim Wallen of Emory University noted, "I think the 'nature versus nurture' question is not meaningful, because it treats them as independent factors, whereas in fact everything is nature and nurture." Wallen said gender differences emerge very early and come about through an underlying preference males and females have for their chosen activities. Femininity is a set of attributes, behaviours, and roles generally associated with girls and women. Femininity is socially constructed, but made up of both socially-defined and biologically created factors. This makes it distinct from the definition of the biological female sex,<ref>[https://web.archive.org/web/20040216042831/http://www.who.int/gender/whatisgender/en/ Gender, Women and Health: What do we mean by "sex" and "gender"?'''] The World Health Organization</ref> as both males and females can exhibit feminine traits. Traits traditionally cited as feminine include gentleness, empathy, and sensitivity,Worell, Judith, Encyclopedia of women and gender: sex similarities and differences and the impact of society on gender, Volume 1 Elsevier, 2001, , though traits