Acid reflux is a possibility, for sure... but what I see WAY more often is too much for babies to cope with.
A baby of the human race would have no 'new' voices or sounds outside the baby's home and village -- not more often than once or twice a month. Babies in our culture see new things every day--sometimes six, eight, 15 times a day, every day, every week, every month.
They also smell more, touch many more new surfaces, and hear more.
Babies brains are not made to deal with the many, many things going on in our 'modern' world...and some kids don't have difficulty ignoring it. Many children have a great deal of difficulty coping, until they mature, or figure out how to cope, or shut down.
If parents know about this, they can limit the babe's exposure intentionally. If parents don't know, they unwittingly create hours and hours of overwhelmed crying every day, thinking life is 'normal'.
at 9:29 PM on Sep. 5, 2009