We should understand the mother and child as a mutually responsive dyad. They are a symbiotic unit that make each other healthier and happier in mutual responsiveness. This expands to other caregivers too.
One strangely popular notion still around today is to let babies ‘cry it out’ when they are left alone, isolated in cribs or other devices. This comes from a misunderstanding of child and brain development.
Babies grow from being held. Their bodies get dysregulated when they are physically separated from caregivers. (See here for more.)
Babies indicate a need through gesture and eventually, if necessary, through crying. Just as adults reach for liquid when thirsty, children search for what they need in the moment. Just as adults become calm once the need is met, so do babies.
There are many long-term effects of under care or need-neglect in babies (e.g., Bremmer et al, 1998; Blunt Bugental et al., 2003; Dawson et al., 2000; Heim et al 2003).
Secure attachment is related to responsive parenting, such as when babies wake up and cry at night