Yes, it is true. When your baby is in physical contact with you much of the day, you are more aware of his needs and you are better able to meet them. While in the sling, baby can feel, smell, and hear his mother or father. The carrier is nice and snug, creating a swaddling effect. The adjustability of most slings allow you to hold your baby in a position that they find most comforting. The parent is often on the move, and the rhythm of this movement soothes baby. In addition, baby often has easy access to the comfort of breastfeeding. The result is, quite clearly, a quieter and more content baby.
Now, certainly a child’s personality is a factor here. Some babies are high needs and some are very easy-going. (I have one of each.) No parenting practice will turn one kind of child into the other. However, babywearing (and generally responsive parenting) can help to mellow the temperament of a high needs child. Your sling baby will have less need to cry than he would otherwise.
In a study, mothers were asked to carry their three-week-olds in their arms or in a soft carrier throughout the day (at least three hours) until the baby was three months old. The carried babies cried measurably less at each stage. At 6 weeks of age (the time when “normal” infants cry the most), the carried babies cried 43% less (and 51% less in the early evening)! The article is found here: Increased Carrying Reduces Infant Crying: A Randomized Controlled Trail.
Also, responding quickly to baby’s cries in the first six months (which babywearing helps you to do) is an investment -- as the following study indicates: In a study done by Sylvia Bell and Mary Ainsworth they spent a year closely observing twenty-six infant-mother pairs in both a laboratory setting and at home. They found that when mothers responded promptly to their baby’s cry in the first six months of life, babies cried less in the second six months. The more demanding babies were those who experienced the delayed response. Infants whose cries were quickly answered requested less holding, enjoyed being held more, protested less when put down, and cooperated more with parental demands than did infants of less sensitive mothers.
If you would like more information about the importance of responding to baby’s cries, read this summary of 19 studies: Science Says: Excessive Crying Could Be Harmful to Babies. If you would like more information about the value of allowing baby to cry in your arms, read this helpful article: Crying for Comfort: Distressed Babies Need To Be Held.