The Real Cost of Children

1. Two cells that are not the same

In 2005 a research group in Paris filmed individual cells of the bacterium Escherichia coli through repeated divisions and followed 35,049 cells in all (Stewart et al. 2005). An E. coli cell is a rod, and it divides across the middle. The two daughters look identical. Each inherits one end of the old cell, the old pole, and builds a new pole at the cut. At the next division one daughter holds an older pole than the other, so each cell can be given an age by counting the divisions its oldest pole has survived.

The cells that inherited old poles grew more slowly, produced fewer offspring and died more often. The authors described the old-pole cell as an ageing parent that keeps producing rejuvenated offspring, and suggested that immortality may be too costly, or mechanically impossible, for living things. Growth in the old-pole lineage fell by about 2 per cent each generation. A later study using a different set-up, with fresh medium supplied continuously, found no decline in the growth of old-pole cells (Wang et al. 2010). The result is therefore contested, and the essay treats it as a finding under particular conditions, not a settled rule.

Budding yeast shows the same split more visibly. Each yeast cell has a finite life, measured since 1959 by counting the buds it produces (Mortimer and Johnston 1959). The parent cell keeps the damaged parts and the daughter starts smaller and fresh, although daughters of very old mothers have shorter lives than daughters of young ones (Kennedy, Austriaco and Guarente 1994).

Division is therefore not a plain copying process that produces two equal cells. Some wear stays with the old cell and the new cell starts again. This happens in single cells that have no childhood, no parent in the human sense and no sex. Repair is expensive, and the disposable soma theory (Kirkwood 1977) holds that an organism has limited resources to divide between keeping itself in repair and reproducing, and that the body is therefore kept in repair only well enough to serve reproduction.

2. Sex without babies

In everyday speech sex and reproduction are one process. In biology they are two. Sex is the combining of genetic material from two individuals. Reproduction is making new individuals. A bacterium can do the first without the second. Bacteria and archaea move DNA from one cell to another by conjugation, transformation and transduction, and none of these makes a new cell (Speijer 2016). Whether they are related to sex in complex cells is unclear.

In complex (eukaryotic) cells, sex by meiosis and cell fusion is present in the last common ancestor of all living eukaryotes, which means it is very old (Speijer et al. 2015; Speijer 2016). Speijer and colleagues describe the general mode of eukaryotic life as clonally propagating lineages of cells with episodic sex, triggered by outside or inside signals. Many single-celled eukaryotes divide for long periods and have sex occasionally. Sex and reproduction became tied together in animals, where each act of sex is also the start of a new individual.

The machinery is costly. Speijer argues that meiosis could evolve only because the energy-producing partner that became the mitochondrion supplied plenty of ATP (Speijer 2016). The shared machinery suggests that meiosis evolved from bacterial transformation, the uptake of DNA from the surroundings. A modelling study argues that the benefit of taking up environmental DNA falls as genomes grow, and that early eukaryotes, with much larger genomes, could keep resisting harmful mutations only by recombining long stretches of DNA, as meiosis does (Colnaghi, Lane and Pomiankowski 2020).

3. What sex costs

A sexual female passes on half her genes to each offspring, and in many species half her offspring are males who do not bear young. An asexual female passes on all her genes to every offspring, and all of them can reproduce. This is the two-fold cost of sex, described by Maynard Smith (1978) and Williams (1975). In the simplest model a clone started by one asexual female would replace a population of a million sexual individuals in fewer than fifty generations (Lively laboratory, Indiana University).

There are other costs. Meiosis breaks up gene combinations that already work. Partners are not always easy to find (Speijer 2016). Mating itself can be dangerous. In a Panamanian pond male túngara frogs gather in choruses and call to females. The calls cost energy and also tell predators where the frogs are, and the fringe-lipped bat, a frog eater, is one of those predators (Ryan 1985). The calls that win mates are also the ones that attract bats.

Sexual reproduction dominates among multicellular animals despite these costs. Biologists infer that the benefits outweigh the costs, and they disagree about which benefit matters most. Because the cost is paid in every generation and the benefit arrives later and unevenly, the question of why sex persists is among the best-known unsolved questions in evolutionary biology.

4. What sex might buy

Sex shuffles genes, and the offspring are different from each parent and from one another. Several explanations for why this pays have been proposed, and they are not mutually exclusive.

Parasites. Parasites adapt to the most common host type in their area. A host lineage that is common becomes the target. Sexual hosts keep producing new combinations, so some offspring are not matched by the parasites' current form. The idea that parasites keep sex going is associated above all with Hamilton (Maynard Smith, in an interview, calls him its main proponent). It is called the Red Queen hypothesis after a name Van Valen (1973) gave to the constant coevolution between species, and the version applied to sex came later. Its best-known test is a New Zealand freshwater snail, Potamopyrgus antipodarum, in which some females reproduce sexually and others clone themselves, and the two kinds often live in the same population. Lively (1987) found that populations with both kinds were more heavily infected by a sterilising worm parasite than populations that were wholly asexual. Later work found that the share of sexual snails and the diversity of clones were both higher where infection was more common (King, Jokela and Lively 2011). In a ten-year study of one population, clones that were common at the start became more susceptible to local parasites over time and some disappeared, while sexual snails stayed more stable (Jokela, Dybdahl and Lively 2009). Parasite adaptation to the common clones appears to stop the clones from eliminating the sexual snails.

Harmful mutations. Sex can remove harmful mutations from a lineage by recombining them away. This idea goes back to Muller (Maynard Smith 1978), and Meselson's laboratory has been exploring it (as reported in Harvard Magazine, 2000).

Changing conditions. Sex can bring together useful new combinations faster than mutation alone, which helps when the environment changes (Fisher 1930). Speijer (2016) describes the common point: reshuffling lets a population quickly get rid of bad combinations and find good ones.

The best-known exception is a group of freshwater animals, the bdelloid rotifers. No male has been found in hundreds of species, and Welch and Meselson (2000) found that their genomes lack the pairs of closely similar gene copies that sex normally produces, which fits many millions of years without it. Estimates of how long range from about 30 to about 100 million years. Asexual lineages arise from time to time, but on the evidence Meselson cites most disappear after a short span in evolutionary terms. A Cornell study found that cultured rotifers can rid themselves of a deadly fungal parasite by drying out completely, and can then be carried by wind to found new populations free of it (Wilson and Sherman 2010). The rotifers escape their parasites in space and time, not through variation.

These findings fit the idea that sex is a defence against disease and change. They do not prove that it is the only reason, and the evidence from one snail and one rotifer group cannot be generalised without care.

What the benefit belongs to. The benefit of sex is often described as a benefit to the species. Evolutionary biologists since Williams (1966) have argued that natural selection acts on individuals and the genes they carry. A benefit to the species would then be a consequence, not the reason a trait spreads. In the snail case the sexual and the clonal females coexist, and nothing in the evidence suggests that either acts for the good of the population.

5. The price of the next generation

Making offspring costs energy, time and risk. Biologists call the way each species divides its resources between itself and its young its life history. A female can put her resources into many small, unprotected young or into a few large, protected ones (Stearns 1992). In amphibians, a comparison of over 800 species found that parental care and egg-laying on land go with larger eggs in smaller clutches (Furness, Venditti and Capellini 2022).

The octopus is an extreme case. Most species reproduce once and die, which is called semelparity. After a female lays her eggs she stops eating, guards and aerates them, and dies about the time they hatch. Removing a gland between the eyes, the optic gland, from brooding females in the 1970s made them abandon their eggs, start eating again and live about six months longer (Wodinsky 1977). Later work showed that after reproduction the gland changes its output of steroid hormones and other signals (Wang and Ragsdale 2018; Wang et al. 2022). For the mother the cost of one clutch is her life.

Most fish and most amphibians give no care after laying eggs. About 30 per cent of fish families show some form of parental care, most often by the male (Gross and Sargent 1985, as reviewed in the Encyclopedia of Fish Physiology). In amphibians a comparison of more than 1,300 species found that just over 400 show some form of care, more than the 10 per cent often quoted, but the majority of species still abandon their eggs straight after spawning (Furness and Capellini 2019). Those species pay in numbers, not in attention: a large clutch, of which few young survive.

Birth and egg-laying are moments of exposure. The Serengeti wildebeest give a measured case. Herds of nearly a million produced about 80 per cent of their young in three weeks (Estes 1976, as cited in a 1977 Nature paper on the lunar cycle and wildebeest mating), and recent counts put the number at up to about 500,000 calves within two to three weeks. One suggested advantage is that predators can only eat so many, so a calf born in the middle of the peak is less likely to be taken. Predators concentrate on the calving grounds, and many calves are still killed in the first weeks. Births in 13 Serengeti species have been compared. Large species in big herds, whose young can stand and run within an hour, bring their young in a tight pulse. Smaller species whose young lie hidden spread their births out (Sinclair, Mduma and Arcese 2000). Both patterns appear to be defences against predators, in opposite forms. The pattern is not universal: in a study of white-tailed deer, whose fawns hide, survival fitted predator satiation better than predator avoidance (Michel et al. 2020).

Hey!, Want To Know: Why Do Animals Sleep at All? considers a parallel: an animal that sleeps spends hours exposed to predators, and the cost has to be offset by a benefit large enough to explain why sleep persists. The same reasoning applies to mating and birth. Sleep as Biology — Adaptive Inactivity and the Flexible Sleeper sets out the biology of sleep as adaptive inactivity.

6. Strategies of numbers

Spending on a few young and spending on many are two ends of a range. A third strategy uses numbers themselves. Several animals gather, travel or breed in huge groups, and the group does for each member something that no single parent could do.

Frogs that breed in a rush. Wood frogs are called explosive breeders. Hundreds gather at a pool for a few nights in early spring, each female lays a mass of 1,000 to 3,000 eggs, and the egg masses are laid together in one communal cluster (Animal Diversity Web; Indiana Herpetological Atlas). In tropical forests, explosive breeding events in a pond have lasted between 24 and 70 hours and are tied to particular rainfall patterns (Ulloa et al. 2019, a study of five ponds in French Guiana). The timing is set by the weather. Short breeding windows suit temporary pools, and the sheer number of eggs and tadpoles makes it harder for predators to take most of them.

Ants that hunt as an army. Army ants of the genus Eciton have no permanent nest. A colony hunts in swarm raids, in which very large numbers of workers move together across the forest floor, and the workers form a living nest, the bivouac, by linking their own bodies around the queen and her brood (Kronauer 2020). The colony alternates between a stationary phase of about 20 days, when the brood develops and the queen lays eggs, and a nomadic phase of about two weeks, when it raids and moves almost daily. The queen can lay tens of thousands of eggs in a cycle, and accounts differ on the figure (AntWiki; Encyclopedia of Life). Small workers tend the brood. When a colony grows too big it splits, with the old queen leading one swarm and a daughter queen the other.

Bees that divide a colony. A honeybee swarm is the colony reproducing itself. The old queen leaves with a large share of the workers, between about a half and two-thirds, perhaps 10,000 bees, while a daughter queen and the remaining workers stay in the old nest. The swarm hangs on a branch while several hundred scouts search for a new nest site, report candidate sites by waggle dance, and settle the choice when enough scouts agree on one site (Seeley 2010). The swarm is a way of founding a new colony in a new place. It also protects the old colony's brood, which stays behind with workers and a new queen.

Termites that defend and disperse. A termite colony has reproductives, workers and soldiers. Workers care for the eggs and young, forage and build, and soldiers defend the colony (University extension publications). Winged adults, the alates, fly out in a swarm, pair up, lose their wings and start a new colony, and the new king and queen tend the first small batch of eggs themselves. Most alates die within days, and only a tiny fraction found a colony, so dispersal flights work as a numbers game. In one study of Coptotermes gestroi, each founding pair carried only enough internal reserves to raise its first few workers, fewer than 15, and had to establish them within about 150 days or die (Chouvenc 2022).

Locusts that move on. Desert locusts live alone or in swarms depending on crowding. Within about two hours of forced crowding a solitary locust becomes gregarious, and a rise in the nerve chemical serotonin causes the change (Anstey et al. 2009). Swarms can contain billions of insects. One of the researchers described the gregarious phase as a strategy born of desperation and driven by hunger, with swarming as a response to find new pasture.

Spiders that fly. Many young spiders disperse by ballooning. They release strands of silk and are lifted into the air, and spiders have been found 4 km up and have dispersed hundreds of kilometres. Experiments show that electric fields in the atmosphere can trigger ballooning and provide lift even without wind (Morley and Robert 2018). Ballooning has evolved in spiders, spider mites and moth larvae (Bell et al. 2005). In temperate regions it is done mainly by the juveniles of larger spiders, such as wolf and crab spiders, and by the adults of small money spiders.

These cases differ. The army ant raid is hunting, the honeybee swarm is colony reproduction, the termite flight and the spider balloon are dispersal, the locust swarm is migration for food, and the wood frog rush is the timing of breeding. They share one feature. The cost of reproduction is spread across numbers, and many individuals pay for the few that succeed.

Ants, bees and termites add a further point. In their colonies most individuals are workers that do not breed, and they feed and tend the queen's young. The cost of raising the next generation falls on siblings. This arrangement is usually explained by kin selection (Hamilton 1964). It means that "no care" cannot be said of all insects. A solitary beetle may give none, and a termite colony devotes most of its members to it.

7. A brain that finishes itself after birth

A bacterium has no juvenile phase and divides at once. Yeast daughters are smaller than their mothers and have a short stage of growth before they divide. In multicellular animals a long period of growth before breeding is common. A young animal that delays reproduction can grow larger, learn more and breed better later, at the price of the risk of dying before breeding at all.

Some young animals are born able to do most of what adults do. A wildebeest calf runs within an hour of birth. Many others are born with bodies and brains that are far from finished, and the finishing is done after birth, through use. How a human brain builds itself from birth to adulthood is the subject of Learning to Survive — How the Human Brain Navigates Opportunity and Danger and, in plain speak, In Other Words... How a Brain Builds Itself — and What That Has to Do With Relationships.

The human case is measurable. Brain volume increased by 101 per cent in the first year of life and by a further 15 per cent in the second, and the cerebellum, a structure that helps coordinate movement, increased by 240 per cent in the first year (Knickmeyer et al. 2008). By age two the brain has reached 80 to 90 per cent of its adult volume. Synapses, the connections between nerve cells, are overproduced early. In the visual cortex their density peaks in the first year after birth and falls to adult levels by about age 11, and the loss of surplus connections depends on visual experience (Huttenlocher and de Courten 1987, as cited in a Journal of Neuroscience study of the early blind). Wiring is therefore completed by the young animal's own activity.

The classic demonstration is in cats. Hubel and Wiesel found in the 1960s that a kitten deprived of sight in one eye during the first three months of life had permanent changes in the circuits of its visual cortex, while the same deprivation in an adult cat had no effect (Wiesel and Hubel 1963; Hubel and Wiesel 1970). Wiring depends on activity during a limited period.

Movement is learned in the same way. Young songbirds learn to sing in two phases. In the first the bird listens to and memorises an adult tutor's song. In the second, which lasts weeks or months, it starts with a babbling-like subsong, moves through a more structured but variable plastic song, and ends with a fixed adult song, using its own hearing to match its output to the memorised model. Human infants show a parallel path from babbling to speech, with similar sensitive periods (Doupe and Kuhl 1999). The skill is a coordination of breathing, voice and hearing, and no one is born with it complete.

The memory involved is of a particular kind. Procedural memory, the memory of how to do things, depends in vertebrates on the basal ganglia and cerebellum, not the hippocampus, and it survives when other forms of memory are lost. The YFL essay Why Memory? — of all things? describes it, including the case of a man with severe amnesia who kept improving at a mirror-drawing task without remembering that he had done it before.

Play as practice. Young mammals spend much of the juvenile period at play. In rats, play fighting emerges shortly before weaning, peaks in the juvenile period and declines with sexual maturation, during a window in which social experience strongly influences brain maturation (Lab Animal, 2026). When juvenile rats were deprived of social play and later returned to company, they showed lasting differences in inhibitory connections in the prefrontal cortex and used simpler strategies in a learning task (Bijlsma et al. 2022). One daily hour of play during the deprivation period partly rescued the learning skills but not the loss of inhibitory synapses. In an earlier study, neurons in the medial prefrontal cortex responded specifically to the experience of play, while neurons in the orbitofrontal cortex responded to the number of peers present (Bell, Pellis and Kolb 2010). These are findings in rats. The studies cited here did not test other species, including people, and how far the findings extend is a separate question. the YFL essay Play—the Brain's Natural Learning Environment discusses play and emotional regulation in children. In Other Words... Play is How the Brain Learns gives the plain-speak account, and Pattern and Purpose: Play Schemas and the Architecture of Early Learning traces how repeated patterns of play reveal the cognitive work of early childhood.

What cannot be born with the animal. Some knowledge belongs to the group and takes a lifetime to gather. African elephants show this. In Amboseli in Kenya, family groups led by older matriarchs were better at telling the calls of familiar elephants from the calls of strangers, and groups with older matriarchs produced more calves (McComb et al. 2001). Only groups led by matriarchs aged 60 or more reacted to the greater danger of male lion roars (McComb et al. 2011). In the 1993 drought in Tarangire National Park, Tanzania, 16 of 81 calves in three family groups died, a rate of 20 per cent against about 2 per cent in normal years. Two groups with matriarchs aged 45 and 38 left the park, apparently in search of food and water, and kept their calves alive. Family groups that stayed suffered heavy calf loss, and calf loss was higher among young mothers than among experienced mothers. The authors suggest that older females remember distant resources from earlier droughts (Foley, Pettorelli and Foley 2008). These studies measure what the adult females know. They do not follow calves as they learn. Long-term observation in Amboseli shows that calves rely on their mothers alone for the first three months, are weaned by about three years although many suckle longer, and spend more time with other females as they grow (Lee and Moss 1986, as summarised by ElephantVoices). Most females have their first calf at 14 or 15, and calves born to mothers aged 7 to 10 are less likely to survive their first year. The presence of older females who help raise calves (allomothers) improves a calf's chance of survival.

Among human hunter-gatherers, body size alone does not explain the long delay before hunters reach peak productivity. Among Tsimane hunters in Bolivia, hunting skills can take another ten to twenty years to develop after adult body size is reached (Gurven, Kaplan and Gutierrez 2006). A comparison of foraging returns in 28 societies found that returns for skill-intensive foods such as tubers and game rise slowly and peak in adulthood, while returns for easier foods such as fruit and shellfish reach adult levels by adolescence (Pretelli, Ringen and Lew-Levy 2022). A chimpanzee has about five years of infancy, five of juvenility and two of youth before it reproduces (Hochberg and Belsky 2013).

Two kinds of explanation compete for the long juvenile period. One is cost: a large brain takes a long time and a lot of energy to grow. Barton and Capellini (2011) compared mammals. Larger-brained species have longer pregnancies, mature later and live longer, and the length of the two phases of maternal investment, pregnancy and lactation, tracks brain growth before and after birth. They found no support for a direct trade-off between brain growth and body growth across species, and concluded that the slow life histories of large-brained mammals follow from the cost of growing the brain. The other explanation is benefit: a long juvenile period pays because wiring, practice and learning pay later. The two are not exclusive. Kuzawa and colleagues (2014), described below, found a trade-off between brain and body growth within human childhood, which is a different level of comparison from the species comparison.

8. Puberty and the gap before breeding

In humans puberty does not mark the end of growth or the start of full reproductive life. Girls reach menarche about nine months after the peak of the adolescent growth spurt. They then have one to three years of adolescent sterility, in which ovulatory cycles are absent or infrequent. Apes have about a year of reduced fertility after the first signs of oestrus. Boys are fertile before the peak of the growth spurt but do not have the build of adult men until about 18 (Bogin 2010, as summarised by the CARTA anthropology resource).

In societies that practise natural fertility, the average time between menarche and the first birth is about four years (Hochberg, Gawlik and Walker 2011, a study of 22 societies). A review from Seoul National University traces the gap to physiology: the hormonal system that regulates ovulation matures slowly, its feedback is incomplete, and it depends on energy balance and nutrition. The review sets out three proposed reasons for the gap, a trade-off between growth and reproduction, a gain in inclusive fitness, and a "beginner mate-search" effect, and it calls for more research, especially among hunter-gatherers (Kim and Park 2025).

Puberty also responds to the environment. In industrial societies the average age at menarche fell from 16.5 years in 1880 to about 12.5 years (Hochberg, Gawlik and Walker 2011). The critical-fat idea holds that menstruation does not normally begin until the body has stored enough fat to meet the demands of pregnancy, about 17 per cent of body weight for menarche on one estimate (Frisch and Revelle 1971; Frisch 1987). Hochberg and Belsky (2013) describe the move from juvenility to adolescence as adaptive plasticity in response to energy resources and other cues. Skeletal evidence from 994 adolescents in four medieval English towns shows puberty beginning at about 10 to 12, as now, but menarche delayed by up to three years, at around 15 for most girls and 17 in London, and the growth spurt in boys ending as late as 21 against 16 to 18 for modern European males. Poor diet, infection and heavy physical work may have contributed (Lewis, Shapland and Watts 2016).

This evidence describes a gap between becoming able to reproduce and being ready to, produced by the same limits that slow growth: a limited supply of energy, a costly brain and a long time needed to learn. The sources reviewed do not describe puberty as a gate held shut so that the young can learn. They describe a dial set by energy, with a delay that may have several benefits, and the balance between them is still debated. What adolescence does to the brain and body is described in plain language in In Other Words... what three significant hormones in adolescence are doing at the front door and Hey!, Want To Know: why your teenager can reject closeness? And it looks like disrespect..

9. Why humans take so long

Humans are an extreme case. They spend almost twice as long in childhood and adolescence as chimpanzees, gibbons or macaques do (Kuzawa et al. 2014, as reported in Science). Kuzawa and colleagues combined brain scans and body-growth data and found that the child's brain uses glucose at its peak in the middle of childhood, at about four to five years, when it takes the equivalent of about 66 per cent of the body's resting energy use. At that age body growth is at its slowest. The authors conclude that slow growth in childhood frees energy for brain development, and that the peak comes after weaning in most small-scale societies. A large part of the cost of the human brain is therefore met by relatives and the group, not only by the mother (Kuzawa et al. 2014; Kuzawa lecture summary, UCLA).

Two theories explain how the costs of human children came to be shared. Hrdy (2009) argues that humans evolved reproduction so costly that mothers needed help from others, and that cooperative breeding shaped human social minds. Hawkes and colleagues (1998) argue that grandmothers' help in feeding weaned children can explain late maturity, early weaning and high fertility, and also the long post-reproductive life of women. Kaplan and colleagues (2000) compared hunter-gatherers with chimpanzees. Hunter-gatherer children survive to age 15 at a higher rate, about 60 per cent against about 35 per cent, and the authors link the human pattern of long learning, high production in adulthood and long life to a diet of high-quality foods that take skill to obtain. Hochberg and Belsky (2013) divide the human pre-adult period into infancy (30 to 36 months), childhood (a further two to four years), juvenility (three to four years), adolescence (three to five years) and youth (about four years), where the chimpanzee has three stages.

Some questions remain open. How much of a long childhood is caused by brain cost, by learning, by the help of others, or by all three? Hrdy, Hawkes and Kuzawa emphasise different elements, and the studies that bear on each use small-scale societies as evidence, not the past of the whole species. The animal side of the same question, why some of the longest-lived animals also take the longest to grow up, is the subject of Why Old? — of all things? and, in plain speak, In Other Words... What Long Childhoods Are For.

10. What the cost says about "child"

In the biology above, a young animal is the result of a chain of costs. A cell has to divide, and some of its wear stays behind. Sex adds a further price. Birth has its own risks. Raising the young takes time, food and attention that are not available for anything else. In most species the cost falls on the parents or on the number of eggs. In colonies of ants, bees and termites it falls on workers that do not breed. In humans it is shared among parents, kin and the wider group.

The idea of a "child" in law and society comes after this. A society that has to feed, protect and teach young members for years has reason to mark out who they are, who is responsible for them, and when the responsibility ends. The age limits of law, such as marriage and majority, are set inside the gap between puberty and full growth and learning, and they are set differently in different places and periods.

Other YFL essays follow those limits in particular cases. The YFL essay Counting a Child's Age looks at how a child's age has been counted and proved, from the Laws of Manu to the registers of nineteenth-century England. The YFL essay Who Decided the Child Was Grown? follows the thresholds set by households, employers, parishes and religion, for girls and boys, rich and poor, and the YFL essay How the Law Moved the Line follows the age limits of United Kingdom law. The YFL piece In Other Words... Who Governed When the King Was a Child? tells how kingdoms were run when the king was a child. The YFL essay Carving the Me asks how much of who a person becomes is theirs to decide. The YFL essay Marvel Children: Who Makes the Wonder? looks at young performers whose abilities were made into wonders by the adults around them. None of them is needed to follow this essay.

The biology above gives one plain result. A young animal is the product of costs that someone has paid, and in people those costs are shared widely and for years. That sharing is a reason, though not the only one, for the limits that societies set around childhood.

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