The tape measures and weighing scales of the Victorian brain scientists have been supplanted by powerful neuroimaging technologies, but there is still a lesson to be learned from historical examples such as these. State-of-the-art brain scanners offer us unprecedented information about the structure and working of the brain. But don't forget that, once, wrapping a tape measure around the head was considered modern and sophisticated, and it's important not to fall into the same old traps. As we'll see in later chapters, although certain popular commentators make it seem effortlessly easy, the sheer complexity of the brain makes interpreting and understanding the meaning of any sex differences we find in the brain a very difficult task. But the first, and perhaps surprising, issue in sex differences research is that of knowing which differences are real and which, like the intially promising cephalic index, are flukes or spurious.