Back in the day, it was common to “sleep train” young babies by simply putting them in their crib and walking away, letting them “learn to soothe” by crying themselves to sleep.

Now, there are modified, or “kinder” versions of this method, but some moms claim letting a young baby cry does damage that can never be repaired.

Who is right?

Well, science is weighing in on the matter, and the answer might surprise you.

Child psychologist Jodi Mindell says that first, you have to be aware that modern “cry it out” techniques don’t involve leaving an infant to cry all night anymore.

“That’s not the reality of what we recommend or what parents typically do. It doesn’t matter if you come back and check on the baby every 30 seconds or whether you come back every five minutes. If it’s your first child you’re going in every 20 seconds.”

Experts also don’t recommend starting too early, but waiting for more than seven months after birth, or until your baby has developed object permanence.

Object permanence mean your baby can figure out that just because you’re not in the room, it doesn’t mean you have literally disappeared.

University of British Columbia pediatric sleep researcher Wendy Hall says that before then, leaving a baby to cry alone is “psychologically damaging.”

“There are a lot of people out there who just put up a shingle and start working with parents and telling them what they should or shouldn’t do, without an understanding of what they’re potentially doing to these babies.”

In fact, whether or not sleep training is healthy at all is a tricky question to answer.

In 2015, Hall recruited 235 families for a study on the effectiveness of sleep training method known as “controlled crying.”

This is when a parent soothes a baby for 2-10 minutes, then leaves them to fuss and hopefully fall asleep on their own.

If they don’t, the parent will return to soothe again, but leave the baby alone for longer periods of time as the evening wears on.

The resulting paper said the results were largely positive.

“Our principal findings (adjusted for baseline) indicated a significant improvement in parents’ perceptions of the severity of the infant sleep problem, reduction in numbers of night wakes by the sleep diary, increase in length of longest night sleep by actigraphy, and improvement in parents’ cognitions about infant sleep, fatigue, sleep quality, and depression in the intervention group compared to the control group.”

All of the successes, however, were parent-reported, which means the results will always be skewed.

The results are how parents perceived their baby was sleeping, not how they were actually sleeping.

“At six weeks, there was no difference between the intervention and control groups for mean change in actigraphic wakes or long wake episodes.”

Which is to say, the babies might not have been sleeping better at all – they may have just learned not to cry.

Many other studies have concluded the same, leaving some parents to worry that even though they’re getting more sleep, they may still be damaging their child.

Some experts want to wipe those worries away, though, saying that the babies are fine and have simply learned to soothe themselves back to sleep.

“Don’t underestimate the abilities of children to self-regulate. Parents can help them learn to self-regulate by giving them opportunities to self-regulate. That’s how you can look at self-soothing. It’s an opportunity to calm themselves down.”

If you’re someone who just doesn’t feel right about leaving your baby to cry for any length of time, don’t worry – sleep training doesn’t seem to have any effect on their personality and development in the long term.

“No evidence that a population-based targeted intervention that effectively reduced parent-reported sleep problems and maternal depression during infancy had long-lasting harmful or beneficial effects on child, child-parent, or maternal outcomes by 6 years of age.”

Studies have also found that the positive effects of sleep training can disappear by the age of two, and most babies will sleep through the night by 20 months, regardless of whether or not they were “trained” to do so.

Like most things when it comes to kids and parenting, your mileage may vary.

Sleep training can be more stressful than interrupted sleep for some people, and every kid’s personality is different.

There is research on the other side that shows benefits for co-sleeping, too, including better and longer sleep for all parties involved, better short-term psychological outcomes, and lower stress for both.

If you’re breastfeeding, there are positive effects for milk supply, too.

So, do what feels right for you and your family.

As long as everyone is happy and healthy, nothing else matters.

If you thought that was interesting, you might like to read about a quantum computer simulation that has “reversed time” and physics may never be the same.