James' Law should really be called James' Laws, as it is in two parts, either of which can be derived from the other. It is quite an obvious rule when you think about it, but because of the subjective nature of those it applies to, they often fail to work it out for themselves.

James' Law, part one, is that all mothers are really kind, except your own. The inverse of this, James' Law, part two, is that all children are really well-behaved, except your own.

It is a very small rule of life, and is a special case of the same principle that gave us 'familiarity breeds contempt', 'absence makes the heart grow fonder', and other such words of wisdom. However, it may be thought that there is some extra element here, because one's parents and/or children behave better when they are with strangers, so one only sees the worst behaviours of one's own family, but the best of other people's.

Who James was, or why he should have this law named after him, is uncertain, but the (admittedly not comprehensive) research I have done suggests that the axiom is a fairly recent one. Anyone who manages to find out the origin of James' Law should please /msg me with this information.