Any more than two children, and they will develop pack instinct
It goes something like this: One moment you are laughing hysterically at your two-year-old's antics in front of the TV, and the next you are weeping into each other's arms that you'll never again experience the cuteness of a toddler in the house because said toddler is your second child and - as we all know - two children is enough.
Two is one each. Two is a second one to keep the first one amused, so you don't have to. Or, if you happen to be royal, two is an 'heir and a spare'.
But perhaps Kate Middleton didn't get the memo, as she has just announced that she is with child for the third time. In so doing, Kate has confirmed what I have long suspected: when it comes to family sizes, more is increasingly, more.