Where you sit in the sibling line-up – eldest, middle, last, or only child – has an impact on your personality and shapes you as an adult. The birth order theory was first proposed a century ago by Austrian physchiatrist Alfred Alder, and has intrigued psychologists (and us) ever since. But exactly how much impact does your birth order have in your life? More than what you might care to think, says psychologist Meredith Fuller. So what we need to know about birth order to understand ourselves better?