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The process takes place only in mammals, as only mammals produce milk. The infant is considered to be fully weaned once it no longer receives any breast milk (or bottled substitute).
|The examples and perspective in this article deal primarily with the United States and do not represent a worldwide view of the subject. (November 2012)|
How and when to wean a human infant is a subject of much controversy. The American Academy of Pediatrics recommends feeding a baby only breast milk for the first six months of its life.  However many mothers find breastfeeding challenging, especially in modern times when many mothers have to return to work relatively soon after the birth of their child.
The American Academy of Pediatrics and the World Health Organization recommend waiting until 6 months to introduce baby food. However, many baby food companies market their "stage 1" foods to children between 4 and 6 months old with the precaution that the food is meant to be consumed in addition to breast milk or formula and is just for "practice". These practice foods are generally soft and runny. Examples include mashed fruit and vegetables. Certain foods are recommended to be avoided. The United Kingdom's NHS recommends withholding foods including those "that contain wheat, gluten, nuts, peanuts, peanut products, seeds, liver, eggs, fish, shellfish, cows’ milk and soft or unpasteurised cheese" until a baby is six months old, as they may cause food allergies or make the baby ill. However, recommendations such as these have been called into question by research that suggests early exposure to potential allergens does not increase the likelihood of allergies.
In many cultures around the world, weaning progresses with the introduction of feeding the child food that has been prechewed by the parent along with continued breastfeed, a practice known as premastication. The practice was important in human history in that it naturally gave a child a greatly improved protein source and also that prevents infant iron deficiency. The prechewing of food also gives the baby long-term immunological benefits through factors in the mother's saliva. However, premasticated food from caregivers of lower socioeconomic status in areas of endemic diseases can result in passing the disease to the child.
No matter what age baby food is introduced, it is generally a very messy affair, as young children do not have the coordination to eat "neatly". Coordination for using utensils properly and eating with dexterity takes years to develop. Many babies begin using utensils between 10 and 14 months, but most will not be able to feed themselves well until about 2 or 3 years of age.
At this point, the mother tries to force the infant to cease nursing, while the infant attempts to force the mother to continue. From an evolutionary perspective, weaning conflict may be considered the result of the cost of continued nursing to the mother, perhaps in terms of reduced ability to raise future offspring, exceeding the benefits to the mother in terms of increased survival of the current infant. This can come about because future offspring will be equally related to the mother as the current infant, but will share less than 100% of the current infant's genes. So, from the perspective of the mother's evolutionary fitness, it makes sense for her to cease nursing the current infant as soon as the cost to future offspring exceeds the benefit to the current infant. But, assuming the current infant shares 50% of the future offsprings' genes, from the perspective of the infant's own evolutionary fitness, it makes sense for the infant to continue nursing until the cost to future offspring exceeds twice the benefit to itself (perhaps less, depending on the number of potential future offspring). Weaning conflict has been studied for a variety of mammal species, including primates and canines.
There is significant individual and cultural variation in time of weaning.
Scientifically, one can ask various questions; some of the most straightforwardly empirical include:
As there are significant ranges and skew in these numbers (some infants are never nursed, or only nursed briefly, for instance), looking at the median (half-way mark) is more useful than looking at the average.
Considering biological measures of maturity, notably investigated by Kathy Dettwyler, yields a range of ages from 2 1/2 years to 7 years as the weaning age analogous to other primates – the "natural age of weaning". This depends on the measure, for example: weaning in non-human primates is often associated with eruption of permanent molars (humans: 5 1/2 to 6 years); comparing duration of nursing to length of pregnancy (gestation time) yields a factor of about 6 in chimpanzees and gorillas (humans: 6×9 months = 54 months = 4 1/2 years); body weight may be compared to birth weight (quadrupling of birth weight yields about 2 1/2 to 3 1/2 years for humans; 1/3 of adult weight yields 5 to 7 years for humans); and similarly for other measures.
The age at which children are normatively weaned can vary significantly between cultures, "from 6 months to 5 1/2 years". (In the book of II Maccabees, 7:27, a mother mentions giving milk to her son for three years as though this was normal.) There is often a cultural expectation in the United States that children be weaned early.
Other studies are possible, as in psychological factors. For example, Barbara Rogoff has noted, citing a 1953 study by Whiting & Child, that the most distressing time to wean a child is at 13–18 months. After this peak, weaning becomes progressively easier and less distressing for the child, with "older children frequently wean[ing] themselves."
In science, mice are frequently used in laboratory experiments. When breeding laboratory mice in a controlled environment, the weaning is defined as the moment when the pups are transferred out of the mothers' cage. Weaning is recommended at 3 to 4 weeks after parturition.
For pet carnivores such as dogs or cats, there are special puppy or kitten foods commercially available. Alternatively, if the pet owner feeds the parent animals home-made pet food, the young can be fed the same foods chopped into small pieces.