I have four (4) children. They range in age from 8 to 14. That means that for the last 8 years I have had four (4) people in my home between the ages of 0 and 14. I am well aware that this number is far too small to represent any kind of statistical sample of the general population - but despite that fact, I feel safe in drawing some conclusions about children.
Observation A - A child has an object - such as a toy, a cup full of milk, or socks (on their feet). Later, the child is unable to locate said object, even if it is pointed out to them.
Conclusion 1: When something a child has in their possession loses contact with that child, it ceases to exist.
This object or possession effectively disappears when the child lets go of it. You or I may see the object fall from the child's hand and fall for a very long time and shatter in a spectacular display of destruction and disorder, and the child WILL NOT NOTICE. And when you call attention to said object, they will be surprised to discover it there in a pool of its own constituent elements.
Observation B - The child pours a cup of milk. The adult, witnessing this, cautions the child not to leave the milk on the table (where it will inevitably cease to exist - and also become cheese). Despite this helpful warning (see Conclusion 3), the child will later deny leaving the cup of very new cheese on the table in the sun.
Conclusion 2: The history of a given object has no bearing on its current condition.
This idea manifests itself in a variety of ways. In the previous example, the child will deny breaking the object - regardless of their shared history with that object. Even if they have carried that cherished possession with them for an entire day, singing songs about it and professing their love for it, when they become angered and hurl it away, Conclusion #1 takes effect, and Conclusion #2 allows them to say - with all true honesty and belief, "I didn't do that!"
Observation C - Child sitting on the couch, pulling socks off of their feet, is warned that socks do not belong stuffed behind the cushions, and nods violently with understanding (while craning to see the TV). Later, when confronted with the 5 socks discovered in the cushions behind the spot where they were sitting, they deny loudly and sincerely having ever placed any sock anywhere. The adult will insist on the child removing the socks to their laundry basket. This is declared to be completely unfair.
Conclusion 3: Cause and Effect are not related.
Observation D - You observe a child armed with a dark crayon traversing a room. No other children are home. Upon entering that room a few minutes later, you discover a new work of art not in the Sotheby's catalog, but sure to be worth $24.95 (for the jug of caustic chemicals which will be required to remove it, and the paint to patch the marred spot on the wall). When asked, the child will nod sagely, and cluck with false sympathy that it must have been Sibling A or B - who have both been at school for 4 hours.
Conclusion 4: The existence of Other Children absolves every child of all blame for everything - ever.
If you have an Only Child, you may not have noticed this phenomenon; that is assuming that you also have no pets, cousins, local playmates, convenient adults (like Dad), or imaginary friends for the Child to blame. They don't have to live with you in order to take the rap.
I do hold out hope that these phenomena are limited to the larval stage of human... despite the existence of Doomsday Believers, Global Warming Deniers, and Supply-Side Economists.