How long does a generation last?

Question

Question:

Generetion, as a time period, is a widespread concept (or perhaps an idea) in History, in Social Sciences, and also in common language. Do historians have some sort of consensus on the time span of a generation? Why?

I would appreciate if you could provide some reliable sources for your answer (more reliable than the ones I present below).

Some previous search results:

1. When searching for "generation" on Stack Exchange History, 695 results are returned. However, there is no single definition of the concept that I could find.

2. Wikipedia has a page for the "social concept" of generation. It says:

A generation is "all of the people born and living at about the same time, regarded collectively." It can also be described as, "the average period, generally considered to be about 20–⁠30 years, during which children are born and grow up, become adults, and begin to have children." [...]

Generation is also often used synonymously with cohort in social science; under this formulation it means "people within a delineated population who experience the same significant events within a given period of time". Generations in this sense of birth cohort, also known as "social generations", are widely used in popular culture, and have been the basis for sociological analysis. Serious analysis of generations began in the nineteenth century, emerging from an increasing awareness of the possibility of permanent social change and the idea of youthful rebellion against the established social order. Some analysts believe that a generation is one of the fundamental social categories in a society, while others view its importance as being overshadowed by other factors including class, gender, race, and education, among others.

3. This other question of Stack Exchange History also uses the idea of a generation spanning a period of 25 years (implicitly, in the title) or a period between 20 and 30 years (explicitly, in the body of the question).

Update:

Additional sources of information:

I did some additional research and found out some controversy with the idea that generations usually lasts between 20-30 years, with 25 years as a good average approximation, as stated (in the aforementioned links) by Wikipedia, by another Stack Exchange History question, as well as in most of the comments to my question.

1. First, this article by Pew Research Center considers:

A generation typically refers to groups of people born over a 15-20 year span, such as the Millennial generation, currently the youngest adult generation.

2. Second, this paper entitled Generation, life course and news media use in Sweden 1986–2011 says:

As a point of departure, the generation analyses use the widely recognized classification of the dutifuls (1926–1945), the baby boomers (1946–1964), generation X (1965–1976) and the dotnets (1977–1995).

That is, it takes four "widely recognized classification" of generations and they range between 12 years (generation X) and 20 years (dutifuls).

3. This other paper named Generation Alpha: Marketing or Science? says:

According to the model made by Strauss and Howe (1991), a generation change takes place in our society around every 20 years, with some signs of cyclicality.

They are referring, in the above sentence, to this book, which I do not have access to verify the quote.

The main goal of this third additional source is to criticize, as unscientific, the approach of a social researcher, Mark McCrindle, who published a book, entitled: "The of XYZ: Understanding the Global Generations". According with Google Scholar this book has over 550 citations. I also do not have access to this book, but this site writes about it:

Australian social researcher Mark McCrindle recently coined the term Generation Alpha to describe the generation born between 2010 and 2020.

He argues that as the world advances, it makes sense to cut the generational boundary to a 15-year maximum. McCrindle also hopes this naming strategy will stick and every 15 years there will be a new generation named successively with Greek alphabet letters.

4. Finally, the third paper mentioned above also makes an interesting introductory statement based on the work of the Sociologist Karl Mannheim:

According to Mannheim, an age group can be considered a generation if they share some immanent attributes, generational consciousness or communal characteristics. For this to happen, three requirements must be met: shared experiences, actual cohesion, common attitudes, and forms of behavior (Mannheim, 1969).

This last statement is also coherent with the concept of generation mentioned in Wikipedia as being used in social sciences. However, apparently, there is an unnoticed growing gap between the biological concept of generation described as, "the average period, generally considered to be about 20–⁠30 years, during which children are born and grow up, become adults, and begin to have children" and the sociological concept of "an age group that share some immanent attributes, generational consciousness or communal characteristics".

The quote of Wikipedia on biological generation length is based on an OECD document which was "retrieved in 2011", but has a broken link. A search for the original document, leaded me to this updated publication of 2019 showing that: "In most OECD countries, the average age at which women give birth now stands at 30 or above".

That is, while biological generation time span has widened to above 30 years, the sociological generations time span apparently has shrunk to under 20 years. This probably has important consequences to social, and therefore, historical dynamics.

If this hypothesis holds, then the average social distance between parents and children (that is, individuals that are one biological generation apart) may be about two social generations. That means the distance on "actual cohesion, common attitudes, and forms of behavior" between parents and their children might be equivalent today as it was between grandparents and grandchildren in the past, potentially leading to new forms of intergenerational conflicts and social polarization.

Answer 1

One could say that a generation-span is the same as the average age of motherhood. A good information source would be census returns, when analysed statistically. One could also use female fertility data by age, if that were available, from which the average age of motherhood can be found. There will be a mean age and a spread (usually defined by the standard deviation). Today, in the UK, this is about 30 +/-8 years. In ancient times the average age of motherhood could have been 26 +/-6 years. From a statistical point of view these numbers can be said to be the same, since the estimates overlap.

Answer 2

I've already commented to say that it should suffice to say how you've defined the value, and if anyone disagrees with it they have a starting point on which base that claim. However, I also have a potential academic source where 25 years has been used in the past.

Sir John Glubb, in 'The Fate of Empires and Search for Survival', explored the rise and fall of states. He managed to approximate the period in which this process happens to a matter of ten generations or 250 years:

One of the very few units of measurement which have not seriously changed since the Assyrians is the human ‘generation’, a period of about twenty-five years. Thus a period of 250 years would represent about ten generations of people. A closer examination of the characteristics of the rise and fall of great nations may emphasise the possible significance of the sequence of generations. ...

This would allow you to at least source the use of 25 years back to someone else if you so desired.

Sir John goes on to describe how in these "sequence of generations" an empire expands, conquers, trades, enriches itself, learns, and then becomes decadent. The ten generations is based on an approximate calculation where he looks at eleven states and their rise and fall with the 250 years/ten generations being a good average that fits most of them.

Answer 3

From William's coronation in 1066 until the Act of Union in 1707, and excluding disputed claimants (Matilda, Louis, Jane), repeat reigns (Henry VI Edward IV), and co-monarchs (Philip simultaneous to Mary) but including the Lords Protector, there were 32 Monarchs of England in a span of 641 years, wielding an average of a trifle over 20 years per reign. Yes there are skips - and there are also siblings. It's a bit of an arbitrary calculation, but at least it's an objective basis from which to begin refinements.

Following the Act of Union to the present, and excluding Anne (reigned 1702-07, 1707-14) as a repeat, there are have been 11 Monarchs of Great Britain in 313 years, giving an average reign length of 28.5 years. This is hard evidence, Elizabeth II's longevity notwithstanding, that the "length of a generation" has increased over the past millennia: in Western Europe at least.

From the standpoint of strictly social history I have seen longer figures used, occasionally 25 years but more often 30 years. This reflects the large family sizes typical in most cultures until quite recent times, with the observation that women giving birth frequently from roughly 20 to 40 are creating an average age of giving birth near about to 30.

So the correct value to use will depend, at a minimum, on both the time period under consideration (generations now are longer than previously) and on the point being made (a longer figure may be more appropriate when considering social history as compared to political history).

Monarchs of England From The Conquest until the Act of Union (1066-1707):

1. William I
2. William II
3. Henry I
4. Stephen
5. Henry II
6. Richard I
7. John
8. Henry III
9. Edward I
10. Edward II
11. Edward III
12. Richard II
13. Henry IV
14. Henry V
15. Henry VI (twice)
16. Edward IV (twice)
17. Edward V
18. Richard III
19. Henry VII
20. Henry VIII
21. Edward VI
22. Mary
23. Elizabeth
24. James I
25. Charles I
26. Oliver Cromwell
27. Richard Cromwell
28. Charles II
29. James II
30. Mary II
31. William III
32. Anne

Monarchs of the United Kingdom From the Act of Union to the Present Day (1707-2020):

1. George I
2. George II
3. George III
4. George IV
5. William IV
6. Victoria
7. Edward VII
8. George V
9. Edward VIII
10. George VI
11. Elizabeth