How long is a generation?

At Saturday's meeting on the Ottawa DNA and genealogy interest group we had some discussion of the length of a generation. It's a topic I covered in the early days of this blog at http://anglo-celtic-connections.blogspot.com/2006/04/timespan-for-generation.html.

I was surprised to learn from the discussion at the meeting that Family Tree DNA use 15-15 years, whereas a group member showed data suggesting 31-32 years. It was also suggested a shorter periods for a generation would be appropriate for earlier times and for working along the female line as women tend to be younger than their spouses.

There's an pertinent article in the now defunct Ancestry magazine at www.ancestry.com/learn/library/article.aspx?article=11152. It points out that a generation has been defined as the age of the parent on the birth of the first child.

For DNA studies you're interested in the span between ancestors along the line through which the specific DNA component was inherited. As its unlikely your inheritance will always be through the eldest child the generation span for DNA purposes will be longer than by the traditional first child definition.

For practical purposes 30 years, or 3 generations per century, are appealing numbers.