Clutch size refers to the number of eggs laid in a single brood by a nesting pair of birds. Several patterns of variation have been noted and the relationship between latitude and clutch size has been a topic of interest in avian reproduction and evolution. David Lack, an ornithologist and evolutionary biologist, was one of the first scientists to investigate thoroughly the effect of latitude on the number of eggs per nest. Since Lack's first paper in the mid-1940s, an extensive amount of research has been conducted in this area. Most bird species show a common pattern of increasing clutch size with increasing latitude. The proximate and ultimate causes for this pattern have been a subject of intense debate involving the ideas of group and individual selection.