In biology and statistics, generation time can be defined as “the average interval between the birth of an individual and the birth of its offspring.”1 In everyday speech we may see this phrase used in exactly this way, as we have during the pandemic when the generation times of different coronavirus variants were making the news (when it comes to catching viruses, this concept translates to the time between being infected and then infecting the next person).2 We also apply the word generation to human beings in a similar way, although this is tied to culturally defining traits or experiences rather than age at parenthood. There is also, however, another sense of it: When something occurred “generations ago,” we’re often talking about generations which are no longer living and which we may not feel a direct connection to. Thus, the mathematics and the experience of generations can sometimes feel like two very different things.
I enjoy using the original mathematical application of generation time to do a bit of very simplified modelling, exploring this sense of “generations ago” in terms of our lived experiences – the inputs into our lives which shape our sense of present and past reality.3 Suppose we extend the idea of our own lived experiences to include the lessons our elders pass directly on to us, perhaps parents and grandparents (and for some, great-grandparents). How long, then, is a generation of that lived experience?
I heard my grandmother’s stories from my birth to her death, with the stories of her direct experience reaching back to her grandparents; that’s a total span of about 150 years of extended lived experience. This could be longer or shorter for others depending on the biological generation times in their individual family histories. In terms of modern population average female generation time and lifespan, the same grandmother’s grandmother unit of experience (GGU) would be about 135 years (see Figure 1). Anyone joining in on this activity can personalise the unit to their own families or any meaningful chunk of time they’d like to model.
Figure 1. Composition of a single GGU based on mean generation times and lifespans
Now the fun part: Mathematically, this excursion takes us out of population dynamics and into the familiar realm of skip counting (counting forwards or backwards by a number other than 1). But in this case, we’re skip counting across history in terms that feel more personal to us. For me, 600 years ago, 1421 in the late Middle Ages, feels like another world. How many GGU – how many of my family’s lived experience generations we’ve just invented – does it take to stretch all the way back there? Just 4?! What about the first Polynesian settlement of Hawai’i? Just 6 GGU. About 17 GGU ago, the khipu symbolic-numerical record system was invented in the ancient Andes, and partway across the world, Pythagoras was born. The Bronze Age began around 35 GGU ago and the Late Stone Age around 273 GGU. The origin of Homo sapiens stretches back 1,333 GGUs, which is a lot, but somehow comprehensible in a way that 200,000 years is not (ignoring, of course, recent evidence suggesting it may be more like 300,000 years). It’s really not much time at all for encompassing all of human existence as we know it. Thinking about our development as a species this way gives me more empathy for our progress or lack thereof – after all, at the rate our sense of context has the opportunity to develop, we’ve had hardly any time at all to respond to staggering changes.
Of course, this is an extremely simple model making lots of unjustified assumptions about the nature of physical and social reality – lifespans change over time, male and female generation times and lifespans can differ, actual life histories of all our ancestors were unique, this conception of lived experience is fairly arbitrary in a historical sense, etc. But in doing this, I can use a very simple mathematical idea in a brief thought experiment to feel infinitely more connected to the past and future of humanity. Who can resist?
- Thompson, J. N. & Post, E. (n.d.). Population ecology. In The Editors of Encyclopedia Britannica, Encyclopaedia Britannica (online). Encyclopaedia Britannica, Inc.
- Thomas, R. (2021). Understanding the generation time for COVID-19. Plus Maths.
- Boodt, C. P. & Mos, L. P. (1990). A hermeneutical analysis of the social-psychological. In W. J. Baker, M. E. Hyland, R. van Hezewijk, & S. Terwee (Eds.), Recent trends in theoretical psychology (Volume 2, pp. 133–144). Springer-Verlag.
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