1. molars-- most kids get their molars, needed for chewing grains and helpful in chewing meats-- between 13 & 33 months old. This is a very simple indication that a constant, consistent source of carbs & fat would be helpful (breast milk is mostly just that: carbs/sugar and saturated fat).
2. brain development- a baby's brain grows incredibly rapidly through the second year of life, and by age 3 has reached 90% of its adult size. The type of growth that particularly characterizes these years is synaptogenisis, or the formation of connections throughout the brain. A toddler's brain actually makes way more connections than he will use, and by adolescence will have "pruned" away about half of these connections. Also ongoing in the infant & toddler years is myelination, which is the coating of brain cells with fat to allow proper transmission of information across the synapses. What nutrient is essential to brain growth & development? Fat. Specifically, saturated fat. Even more specifically, cholesterol. Breast milk is an amazing source of saturated fat. While cow's milk (and this is an average which varies by breed) has about 8 g of fat, and 33 mg of cholesterol per cup, human breast milk has around 11.2 g of fat, and 43 mg of cholesterol per cup (this also can vary from mom to mom, and mom's diet is a part of that)! Also, the brain is a huge consumer of glucose, which is supplied in breast milk in the form of lactose (carbs make up about 40% of the calories in breast milk). A toddler's brain still needs lots of sugar and fat, and mom's milk is still the perfect source of those through the second year of life. In fact, the longer a mom nurses, the more fatty & energy-dense her milk gets, providing more calories & fat per ounce! (isn't that amazing!? breast milk really is designer custom-made food!) Could this be why IQ scores go up the longer a child is breast-fed?
Most students of the brain note a difference between 2 & 3 years in terms of brain development, rather than between 1 & 2, whether one is studying Erickson's "Psychosocial," Piaget's "Cognitive Development,"or physiological "brain development" stages, or critical periods in language acquisition.
3. immune factors- this is a complex subject that is still not wholly understood even by experts... so please excuse this extreme simplification of the topic: it seems that there are 3 "arms" to a mature immune system: cell-mediated (Th1), humoral (Th2), and regulator (Th3) immunities. Th1 tends to respond to danger in our cells with inflammation (white blood cells), Th2 tends to respond to danger outside our cells with antibodies, and Th3 keeps the other two in check. As Dr. Russell Blaylock (MD) explains:
"If a virus invades, it quickly switches to the Th1 phase, which allows immune cells to secrete a group of cytokines that kill viruses. [...] At other times, the immune system needs a whole different set of immune signals and cells, which are supplied by the Th2 phase. The Th2 phase favors the production of antibodies, mainly supplied by B-cells, but in general they reduce immune reactions."If left alone, babies start to make antibodies on their own around 6 months (artificially-fed babies do get antibodies earlier) and the mature immune system develops by age 3. However at birth babies depend largely on their humoral (Th2) system-- this keeps them from being flagged as an invader by Mom's immune system pre-birth, allows good bacteria to colonize their gut, and also helps keep their brains growing without any inflammation to endanger it. (Think of how rare, and therefore how serious, it is when a young baby gets an infectious fever-- babies just aren't wired to get fevers, which are a form of inflammation.) So, how can a baby fight infection if everything in its system is trying to avoid inflammation to allow his brain & nervous system to develop as much as it can? Breast milk again. A mother's milk will be custom-made with antibodies for whatever germs she has been exposed to-- and with as much kissing and snuggling as moms naturally do with their babies, by day's end, she's been exposed to whatever he's been exposed to. Continuing to supplement baby's immune system with maternal antibodies as long as his brain is rapidly growing and his immune system is still largely in Th2 mode (to protect that brain) makes complete sense-- that would mean breast-feeding for 2-3 years, not 1-2.
4. hints from other mammals- looking at the weaning ages in other mammals and adjusting them for various factors (like gestation, permanent tooth eruption, age of puberty, etc.) is a fascinating study, and indicates a "natural weaning age" for our species between 2 and 7 years. Definitely not before 1 year.
5. child spacing- [[I'm not sure which is the "chicken" and which is the "egg:" if we naturally nurse 2-3 years and that helps us space our kids ideally, or if it's designed so that we generally have another child 2-3 years after the first, and thus wean around that time... either way God's system seems to have the two interrelated somehow.]] Much research indicates that the "ideal" spacing between pregnancies is 2 years, from a purely nutritional point of view. In developing countries, breast feeding is an effective form of child spacing known as "LAM" (lactational amenorrhea), delaying the onset of ovulation for an average of 15 months, or even the duration of breast feeding in some women. [Why doesn't this work in our own society? My own theory is that it has to do with all the hormones we ingest constantly, from our food (especially milk, meat & soy, which is in most processed foods), water and even the plastics in our lives!] While some women are able to "tandem nurse" (nurse through pregnancy and then nurse two children of different ages), many find pregnancy a natural weaning point, or choose to wean in order to "restock" before conceiving again. It seems logical to expect 2-3 years of nursing one child before another child would naturally follow...
I want to restate that any breast milk is a gift. To the moms who nursed to 1 year, awesome. You gave your baby a tremendous start in life. To moms who nursed to 6 months, yes!! You supplied your baby with antibodies until her body started making her own! To moms who nursed 6 weeks; excellent. Your baby got not only "liquid gold" (colostrum) but also got his metabolism correctly calibrated. To the moms who nursed 3 days, you sealed your baby's intestinal wall and gave her a mega-dose of probiotics... kudos especially go to those moms who persevered through pain and/or difficulties to get to whatever milestone you reached!!!
My own "takeaway" as I've waded through so many aspects of this topic has been to aim to nurse well into toddlerhood --whether that means 18 or 24 or 36 months will vary by situation, and by child-- just as each child learns to walk & talk at different ages, so they also get their molars at varying ages, their brains develop at different paces and their immune systems switch from Th2-dominant to a mature balanced system, all at different ages. Follow your body, your child's body, and your own gut... and pray. How many of us have prayed about when to wean? But God invented breast milk, just as He controls immunity and molar eruption. :)