As psychologist Johnny L. Matson observes, using the toilet can be a complex process to master, from the ability to recognize and control bodily functions, to the skills required to carry out proper hygiene practices, the requisite dexterity to dress and undress oneself, and the communication skills to inform others of the need to use the toilet.: 2–3  Usually around one year of age, a child will begin to recognize the need to evacuate, which might be observed through changes in behavior immediately prior to urination or defecation.  Although they may recognize the need, children younger than 18 months may not yet be able to consciously control the muscles involved in elimination, and cannot yet begin toilet training. While they may use the toilet if placed there by a parent at an opportune time, this likely remains an involuntary, rather than a conscious process.: 25  This will gradually change over the course of many months or years, with nighttime bowel control usually the first to manifest, followed by daytime control, and nighttime bladder control normally last.: 26 Toilet training practice may vary greatly across cultures. For example, researchers such as Mary Ainsworth have documented families in Chinese, Indian, and African cultures beginning toilet training as early as a few weeks or months of age.: 1–2 : 216  In Vietnam, toilet training begins shortly after birth, with toilet training complete by age 2. This may be mediated by a number of actors, including cultural values regarding excrement, the role of caregivers, and the expectation that mothers work, and how soon they are expected to return to work following childbirth.In 1932, the U.S. Government recommended that parents begin toilet training nearly immediately after birth, with the expectation that it would be complete by the time the child was six to eight months of age. However, this shifted over time, with parents in the early 20th century beginning training at 12–18 months of age, and shifting by the latter half of the century, to an average of greater than 18 months. In the US and Europe, training normally starts between 21 and 36 months, with only 40 to 60% of children trained by 36 months.Both the American Academy of Pediatrics and the Canadian Paediatric Society recommend that parents begin toilet training around 18 months of age so long as the child is interested in doing so. There is some evidence to suggest that children who are trained after their second year, may be at a higher risk for certain disorders, such as urological problem or daytime wetting. There is no evidence of any psychological problems resulting from initiating training too early.: 83  In a study of families in the United Kingdom, researchers found that 2.1% began training prior to six months, 13.8% between 6 and 15 months, 50.4% between 15 and 24 months, and 33.7% had not begun training at 24 months.: 83 The majority of children will achieve complete bladder and bowel control between ages two and four.: 3 : 162  While four-year-olds are usually reliably dry during their waking hours, as many as one in five children aged five will occasionally wet themselves during the night. Girls tend to complete successful training at a somewhat younger age than their male peers, and the typical time period between the beginning and completion of training tends to vary between three and six months.