Whena baby cries it seriously reduces their oxygen their supply and subsequently harms their ability to send vital chemical signals to their body. Therefore if a child is routinely left to cry, you could be damaging their physical development, as well as thir emotional development. This has been particularly linked to heart issues (such as heart attacks) later in life. You should particularlynever purposefully leave a baby to cry who is under 3 months, as this is an intense period of cerebral devlopment and could damage their physical develoment. Further, it could damage them emotionally- did you know small babies need constant reassurance that they even exist. Imagine if you were crying because you doubted your existence and nobody came- how would that affect you pscychologically and emotionaly? Further, for a young baby in particular, crying for a long period can use up vital calories meaning that will then begin to cry for food (regardless of what they were crying for to start with). All babies cry and they cry instinctively to communicate. It is fine to let a baby cry for a short amount of time as you make your way to them, but you should never leave your child to cry, on purpose for a period of time. Particularly when that baby is less than 3 months old.