This post is just to provide some additional data regarding the "length" or "duration" of a generation. As seen in yesterday's post (which criticises Y chromosome mutation rates), calculations are based on a 25 year generational interval, which when input into the formulas used to calculate the ages of Y chrmosome lineages will give an incorrect date if, (and that is what we will clarify in this post) generations are either longer or shorter than 25 years.
Generations last more than 25 years
A long term study by Nancy Howell among the !Kung of Namibia and Botswana revealed that this contemporary hunter-gatherer group of people are relatively old at the time of bearing children: For women it averages 25.5 years, but for men (and this is important when it gets down to Y chromosomes): 31 to 38 years averaging 34.5 years. These people are very similar to the pre-agricultural society of our distant ancestors. 
A paper (Matsumura and Forster, 2008)  found that, among Eskimos, the father-son interval is 32.1 years. And point out (bold face is mine) : "The majority of the previous studies assumed that the generation time for mitochondrial DNA and Y-chromosomal DNA is 20 and 25 years, respectively (e.g. Harpending & Rogers 2000). We suggest that a higher value, 25–30 for mtDNA and 30–35 years for Y-chromosomal DNA, should be used in genetic inference." .
Fenner (2005)  indicates a male generation length of 31 to 32 years. 
So, it seems that 25 is too short a time for male generations, the value is at least 30 years, and perhaps higher. In polygynoous societies, older men would have monopolized women and the generation length have been even longer.
 N. Howell, (2009). Demography of the Dobe Kung. Transaction Publishers.
 Matsumura S, Forster P., (2008). Generation time and effective population size in Polar Eskimos Proc. R. Soc. B 7 July 2008 vol. 275 no. 1642 1501-1508
 Fenner JN, (2005) Cross-cultural estimation of the human generation interval for use in genetics-based population divergence studies. American journal of physical anthropology 128. doi: 10.1002/ajpa.20188
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