How Long Is a Generation?
By Donn Devine, CG, CGI
We often reckon the passage of time by generations, but just how long is a generation?
As a matter of common knowledge, we know that a generation averages about 25 years—from the birth of a parent to the birth of a child—although it varies case by case. We also generally accept that the length of a generation was closer to 20 years in earlier times when humans mated younger and life expectancies were shorter.
In genealogy, the length of a generation is used principally as a check on the credibility of evidence—too long a span between parent and child, especially in a maternal line, has been reason to go back and take a more careful look at whether the evidence found reflects reality or whether a generation has been omitted or data for two different individuals has been attributed to the same person. For that purpose, the 20- and 25-year averages have worked quite acceptably; birth dates too far out of line with the average are properly suspect.
But now, researchers are finding that facts differ from what we’ve always assumed—generations may actually be longer than estimates previously indicated.
Several recent studies show that male-line generations, from father to son, are longer on average than female-line generations, from mother to daughter. They show, too, that both are longer than the 25-year interval that conventional wisdom has assigned a generation. The male generation is at least a third longer; the female generation is about one-sixth longer.
As early as 1973, archaeologist Kenneth Weiss questioned the accepted 20- and 25-year generational intervals, finding from an analysis of prehistoric burial sites that 27 years was a more appropriate interval but recognizing that his conclusion could have been affected if community members who died away from the village were buried elsewhere.
Why Age Matters
In a more-recent study regarding generation length, sociologist Nancy Howell calculated average generational intervals among present-day members of the !Kung, contemporary hunter-gatherer people of Botswana and Namibia whose lifestyle is relatively similar to that of our pre-agricultural ancestors. The average age of mothers at the birth of their first child was 20 years and at the last birth 31, giving a mean of 25.5 years per female generation—considerably above the 20 years often attributed to primitive cultures. Fathers were six to 13 years older than mothers, giving a male generational interval of 31 to 38 years.
A separate study, conducted by population geneticists Marc Tremblay and Hélène Vézina, was based on 100 ascending Quebec genealogies. Researchers found a generational interval, based on the years between parents’ and children’s marriages, to average 31.7 years, and they determined that male generations averaged 35.0 years while female generations averaged 28.7 years.
Biological anthropologist Agnar Helgason and colleagues used the Icelandic deCODE genetics database to arrive at a female line interval of 28.12 years for the most recent generations and 28.72 years for the whole lineage length. Male line lineages showed a similar difference—31.13 years for the recent generations and 31.93 years overall. For a more mathematically appealing average, Helagason and fellow researchers recommended estimating female generational line intervals at 30 years and male generational intervals at 35 years, based on the Quebec and Iceland studies.
What does this mean to the genealogist? When assigning dates to anthropologically common ancestors 50 or more generations in the past, using the “accepted” 20 or 25 years as a conversion factor can produce substantial underestimates of the time interval.
For my own purposes, however, given the imprecision of the various results and my own need for an estimate that lends itself to easy calculation, I decided that three generations per century (33 years each) for male lines and 3.5 generations per century (29 years each) for female lines, might work better when I needed to convert generations into years.
To check the accuracy of my values, I decided to compare the generational intervals from all-male or all-female ranges in my own family lines for the years 1700 to 2000. I was pleasantly surprised to see how closely the intervals agreed with the estimates I was using. For a total of 21 male-line generations among five lines, the average interval was close to 34 years per generation. For 19 female-line generations from four lines, the average was an exact 29 years per generation.
In genealogy, conclusions about relationships are subject to change whenever better evidence is discovered. Similarly, it’s the nature of the physical and biological sciences that current understandings are subject to change as more data becomes available and that data’s interpretation becomes more certain. So, for now, when genealogists want to convert generations to years and create probable date ranges, using an evidence-based generational interval—like Helagason’s 30 and 35 years or one that you’ve developed based on your own family history research—may be the best solution.
Donn Devine, CGSM, CGISM, a genealogical consultant from Wilmington, Delaware, is an attorney for the city and archivist of the Catholic Diocese of Wilmington. He is a former National Genealogical Society board member, currently chairs its Standards Committee, is a trustee of the Board for Certification of Genealogists, and is the administrator for Devine and Baldwin DNA surname projects.