Babies who continue to cry excessively for no obvious reason can go on to have difficulties in childhood, according to a new study.
One in 20 babies cried excessively
Excessive, uncontrolled crying that persisted beyond three months of age was linked with behavioural problems and lower IQ at the age of five.
The US National Institutes of Health study, in Archives of Disease in Childhood, supports prior UK findings.
Experts said most crying was normal and parents should not be unduly concerned.
The NIH team, working with researchers at the Norwegian University of Science and Technology, looked at 327 babies and their parents.
They assessed the babies' crying patterns at six and 13 weeks of age and whether or not the crying could be explained by simple colic.
When the children were five years old, the researchers assessed their intelligence, motor abilities and behaviour.
The children who had continued to cry beyond three months of age as infants, which was not due to colic, had intelligence scores (IQs) nine points lower than the other children studied.
Prolonged crying was also linked with poorer fine motor abilities, hyperactivity and discipline problems in childhood.
In 2002, a team of UK researchers, led by Professor Dieter Wolke at Bristol University, found children who had cried excessively as babies, beyond three months, were 14 times more likely to develop attention deficit hyperactivity disorder (ADHD) and do worse at school as eight year olds.
Professor Wolke said: "This confirms what we found.
"Now there really is more certainty there is really something going on."
He believes the core of the problem is one of under-regulation.
"With ADHD you can't regulate your attention. You can't concentrate, for example. The same thing is happening with crying.
Picking up problems early
"With these babies, it may be that their brains are built in a way that they have problems regulating themselves. They don't learn to calm themselves down, even with the best parenting."
He pointed out that most crying in babies was completely normal, and that this excessive crying linked to later problems occurred in only about 2-5% of cases.
"But that's still quite a lot of children - between 14,000 and 35,000 children per year in Britain alone," he said.
He said it might now be possible to pick up ADHD as early as six months and intervene.
"These babies, because they are under-regulated, they need incredibly regular routines.
"Some babies you can take to noisy parties and they will sleep and won't cry. But these babies, any change to their routine and they can't cope.
"A very strict, regulated parenting pattern but which is warm and loving seems to reduce the amount of crying.
"What we really need now is a good randomised controlled trial on a large scale to look long term whether intervention has an effect."
Heather Welford from the National Childbirth Trust said: "This is very interesting...and if their findings are correct could affect one in 20 parents.
"Those parents must be having a very difficult time.
"It's very wearing to be looking after a baby who cries a lot, who is not giving you the positive feedback that you need as a parent to let you know that you can tune in to your baby and reassure them."
She said it might be that the crying hampered bonding between the parents and the baby and this contributed to the difficulties in later life.
"We know that a good close attachment between parents and babies is very important for intellectual and emotional development."
She emphasised that in most babies, crying was normal and stopped by about three months and that support was available for parents.