[Piers]

Nature is full of semi linear trends. Trends that are linear for a significant part of their range, but non linear outside that range. This is because curves often look linear over a small portion of their range, ever seen the approximation x~sin(x) for small x.

So when I see a naturally occurring scatter graph that a linear regression will closely fit, my initial assumption is that we are looking at a curve that has a semi linear part rather than a strictly linear relationship. The tendency to assume linear relationships reflects our tendency to simplify complex patterns so they can fit into our limited ability to understand rather than an underlying feature of process being analysed.