"How can you expect to hold on to them later in life, if you begin their lives by pushing them away?" Abbott (1992)
The idea that a baby should be expected to sleep in a room all alone is a particularly unusual notion that became prevalent during the course of the twentieth century in Anglo-American societies. Hardyment (1983) describes this as a consequence of increasing wealth that altered living conditions following the Industrial Revolution. When domestic space for separate sleeping locations was coupled with the popularity of behaviourist child-rearing strategies in the 1920s and 1930s -- emphasising the desirability of self reliance in children, and advocating the witholding of affection by parents for fear of 'spoiling the child', solitary infant sleep became a socially desirable goal. Synonymous with 'progress' and 'upward mobility' the provision of a separate nursery for a new infant became obligatory in middle-class households -- and within the space of a generation, was the norm for night-time infant care. Just a few decades later in the US, UK, and parts of Europe, the moment of birth is now commonly viewed as the beginning of autonomy for a baby who is no longer physically connected to the mother; early sleep independence is a developmental goal aspired to by parents, and towards which infants are pushed.
For most of the world's cultural groups, however, motherhood is typified by constant close contact with her baby -- in fact to separate a baby from its mother for sleep is considered abusive or neglectful treatment (Morelli et al 1992) and private bedrooms for children are the exception rather than the rule around the world (Jenni 2005). Even within Europe there is much variation; Italian parents were reported by Wolf et al (1996) to prefer having their infants sleeping in their rooms, and to consider the American custom of putting children to sleep in separate rooms to be 'unkind' -- although some sub-cultures within the US (such as rural Appalachians of Eastern Kentucky referred to by Abbot in the quotation above) also emphasise how family solidarity is reinforced by sleeping arrangements in childhood and infancy.
Given the novelty of solitary infant sleeping arrangements, it is unsurprising that they are now shown to be detrimental to the health of babies.
Not only has an increased risk of SIDS been strongly associated with solitary infant sleep, but we now also know there are other consequences for babies who are left in rooms alone to 'sleep'. For instance Hayes et al (1996) found that infants and children who were solitary sleepers had a much stronger attachment to a security object and were more likely to be disturbed by that object's absence than infants who did not sleep alone. In a recent study Tollenaar et al (2012) examined the stress reactivity of 2 month old infants who slept alone or in proximity to their parents. Infants with a solitary sleeping arrangement in their first month of life showed a heightened cortisol response to a mild physical stressor (a bathing session) at 5 weeks. The authors suggest that solitary sleeping in the first month of life is associated with heightened sensitivity of the HPA-axis to a mild stressor, possibly due to less nocturnal parental availability as external stress regulator.