The most likely explanation is that the mother's heart is on the left side and, by holding the baby in her left arm, she is unconsciously bringing her baby closer to the sound of the heart beat. This is the sound the baby heard when it was inside the mother's womb and which is therefore an association with peace, comfort and security.

　　Tests were carried out in nurseries where some babies were played the recorded sound of a human heart beat, and, sure enough, went to sleep twice as quickly as the others. We also know that the sound of mother's heart is quite audible inside the womb and that the unborn baby has a well­developed hearing.

　　It is interesting that fathers show less of this left­side bias (偏爱) than mothers, suggesting that the human female is better programmed than her partner for carrying a baby. Alternatively, she may unconsciously adjust her holding behavior to make baby fell more secure. Some new observations on our closest animals' relatives, the chimpanzees and gorillas, have revealed that they also show a strong bias for holding their babies on the left side. The precise figures were 84 percent for chimpanzees and 82 percent for gorillas, remarkably close to the human percentages.

　　Recently a possible additional value in cradling babies on the left side has been suggested. It has been pointed out that, because, the two sides of the brain are concerned with different aspects of behavior, it is possible that the mother, in cradling the baby to her left, is showing the baby her "best side"．It is claimed that the emotions are expressed more strongly on the left side of the human face and that she therefore gives the baby a better chance to read her emotional mood changes as it gazes up at her. Furthermore, the mother's left eye and ear are more tuned into emotional changes in her baby than her right eye and ear would be. So in addition to the baby's seeing the more expressive parts of its mother, there is further advantage that the mother herself is more sensitive to the left­held baby. This may sound far­fetched, but just possibly, it could provide a slight extra benefit for those mothers displaying the strange one­sided bias when cradling their infants.

　　How does a bias occur? Do the mothers have an instinctive preference for it, or do they learn it by trial and error, unconsciously adjusting the position of the babies until the babies are calmer? The surprising answer is that it seems to be the baby not the mother who controls the bias. Observation of new born infants when they were only a few hours old revealed that they come into the world with a pre­programmed tendency to turn their head to the right. If the new born baby is gently held in a dead central position and then released, it naturally swings to the right far more often than to the left. This happens in nearly 70 percent of babies. This may be only a part of explanation, because the holding bias is 80 percent not 70 percent, but it adds a further intriguing chapter to the story.

语篇解读：本文讨论的是为什么大多数母亲都是左手抱婴儿。