A child forsaken, waking suddenly,
Whose gaze afeard on all things round doth rove
And seeth only that it cannot see
The meeting eyes of love
—George Eliot
Some readers may be horrified at picturing a baby's mind as made up of nearly separate agencies. But we'll never understand how human natures grow without some theories for how they start. One evidence for separateness is how suddenly infants switch from smiles of contentment to shrieks of hunger-rage. In contrast to the complex mixtures of expressions that adults show, young children seem usually to be in one or another well-defined state of activity — contentment, hunger, sleepiness, play, affection, or whatever. Older children show less sudden mood changes, and their expressions suggest that more different things are happening at once. Our minds may thus originate as sets of relatively simple, separate need machines. But soon enough each becomes enmeshed in all the rest of our growing machinery.
How should we interpret an infant's apparent single-mindedness? One explanation of those striking shifts in attitude is that one agency attains control and forcibly suppresses the rest. Another view is that many processes continue at once — but only one at a time can be expressed. It would be more efficient to keep the whole array of proto-specialists at work. Then each would be more ready to assume control in case of an emergency.
What would be the advantage in a mechanism that makes a baby conceal that mixture of emotions, expressing only one of them at a time? Perhaps that artificial sharpening promotes the child's welfare by making it easier for the parent to respond to whichever problem has the greatest urgency. It's hard enough to know what infants want, yet think how much harder it would be if they confronted us with complicated expressions of mixed feelings! Those infants' very lives — and, in turn, our own lives — depend upon their expressing themselves clearly. To achieve that clarity, their agencies must be equipped with powerful cross-exclusion devices to magnify small differences that make it clear which needs come first. This leads to simple summaries — which manifest themselves as drastic changes in appearance, voice, and mood that others can interpret easily. And this is why, under circumstances in which adults merely frown, babies tend to shriek.
Given that those signs are clear, what forces us to respond to them? To help their offspring grow, most animals evolve two matching schemes: communication is a two-way street. On one side, babies are equipped with cries that can arouse parents far away, out of sight, or sound asleep — for along with sharpening those signs, cross-exclusion also amplifies their intensity. And on the other side, adults are made to find those signals irresistible: there must be special systems in our brains that give such messages a high priority. To what might those baby-watching agents be connected? My guess is that they're wired to the remnants of the same proto-specialists that, when aroused, caused us as infants to cry in the first place. This leads adults to respond to babies' cries by attributing to them the same degrees of urgency that we ourselves would have to feel to make us shriek with similar intensity. This drives the babies' caretakers to respond to their needs with urgent sympathy.