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I've noticed that with most infants (say, around 6 months old, give or take), when presented with an object, there are 3 main things that they'll do with it:

  • look at it
  • put it in their mouth
  • hold it in their hand(s) and stiffly jerk their arm(s) up and down (banging the object on whatever's in front of them)

It's this third behavior that I'm curious about. Why do they do this? (I imagine that the stiff, straight arm and jerky, banging movements come from a lack of fine motor control, but I still don't see what the ultimate goal is.)

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1's fun! Try it! – DA01 Sep 25 '13 at 5:49
up vote 11 down vote accepted

They are determining some of the properties of the object:

  • can I lift it?
  • how much does it weigh?
  • is it soft or hard?
  • what kind of noise does it make when shaken, or when banged against something?
  • it is symmetrical, or is one end heavier than the other?
  • what do my parents feel about my doing this with it?

The banging may be accidental - you give them something when they're sitting in a high chair, it will get banged against the tray - or part of the exploration. Babies have a limited set of "tests" they can run, but the three bullets in your question cover most of the senses and provide plenty of information about the new thing.

Generally, babies want to learn about everything they interact with. But if some of the tests delight them - the item feels good, tastes good, makes a fun sound - then they will repeat that action with that item and we say they are now playing with it - and we generally approve.

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All the moreso if some of the tests get a reaction from the parents. – James Bradbury Sep 24 '13 at 19:22
Sure, I understand that they want to explore the object. But why do pretty much all babies (from what I've seen) perform this exact same movement? They could just as easily wave their arms left-and-right instead of up-and-down. – Kricket Sep 25 '13 at 8:09
Try it yourself. Up and down uses the same muscles in each arm. Back and forth requires more co-ordination or the arms will oppose each other. (I've seen a 6-7 month old in a self tug of war for an item held with both hands; it's funny.) Also back and forth is harder - that's why we chop wood etc up and down, we can get more movement into the axe for the same effort. – Chrys Sep 25 '13 at 11:22

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