Bernard H. Bichakjian calls this particular feature paedomorphosis which refers to a biological alternative in which aspects of infanthood continue to be inherent to mature aspects of an organism. For example infant facial features are recognizable in adult humans. His theory suggests that linguistic paedomorphic alternatives have selective advantages. Based on the research of LeVay Wiesel and Hubel, Bichakjian [6] states:
It is a known fact that language plasticity like other types of cerebral plasticity is not a potential held constant during a given period of time, but a decreasing disposition during a specific ontogenetic span. Therefore the sooner the neural pathway of a given linguistic feature is laid out, the stronger it is. Better established neural pathways are not only less impervious to negative somatic and psychological conditions (e.g. fatigue or insecurity) and pathological processes (degenerative or accidental types of aphasia), but also more economical in their regular use, since their activation requires less energy. Given the importance of early-established neural pathways, the shift to earlier-acquired linguistic features now becomes a development towards more advantageous alternatives. -Bernard Bichakjian, 1995, p.53-
Basically, vocables that are readily available to children will tend to linger in the language because they are in use earlier in children speech. Bichakjian has surveyed families of indo-european languages and has concluded that the trend from old forms to newer forms in languages occurs as a selection for paedomorphic alternatives. These selections happen in stages, and there are different strategies that support these observations.