Computer simulations explore how cultural practices can "hitchhike" the diffusion of beneficial innovations

What will you have for your Christmas dinner? A US-style stuffed turkey, a British roast dinner topped off by an inevitable mince pie, perhaps oysters, smoked salmon toasts and a “bûche de noël” like many French enjoy? If you are in Japan, you may prepare yourself to queue for a large bucket of fried chicken, or you may not celebrate Christmas at all.

And if you are the one preparing this dinner, who taught you what to cook and the little secrets for perfectly roasted vegetables? Was it your mother-in-law, your paternal grandfather? Do you blindly follow their recipes by the letter, or do you add some of those warm spices that you used to love as a child that instantaneously bring you back home?

These seemingly trivial habits may, in fact, tell us a lot about how past societies have evolved over time. This could even be more important during episodes of interaction between different cultures when the opportunity for learning new practices becomes frequent. The dispersal of farming is perhaps the most intriguing and noteworthy example when quickly growing populations of migrant farmers began to interact with communities of local hunter-gatherers in different parts of the world.

In these contexts, cultural practices like the ones described before may spread along with the rapidly growing population. This process is often called ‘Cultural hitchhiking’, whereby certain cultural practices can "hitchhike" the diffusion of a key innovation such as farming. Culinary traditions such as the beloved British roast dinner and its Neolithic equivalent hardly survive the millennia spent in the archaeological record, but Dr Crema and other members of the ENCOUNTER project team are developing techniques to detect if farmers’ twist to cook the best sticky rice indeed spread within the Japanese archipelago.

Cooking isn’t the only evidence archaeologists can follow to track cultural hitchhiking. Other traditions, such as pottery decoration, specific tool shapes or settlement layouts,  have better resisted the ages and are a great source of information to identify instances of cultural hitch-hiking.

But none had yet investigated the impact of all these processes taken together. What if the migrating farmers, when joining new communities, were faced with a horde of grumpy uncles who would never compromise on what they see as the ideal time to cook rice? Will the migrant's cooking habits disappear, even if they are growing in number? What if only women were migrating through intermarriage, and they were the ones teaching the next generation how to cook? Will their recipes spread, or will they learn the technique used by older ladies in their new communities?

To answer these questions, cultural evolution modeller Simon Carrignon, quantitative archaeologist Enrico Crema, mathematician Anne Kandler and archaeologist Stephen Shennan developed a simulation model to understand what promotes or hinders cultural hitchhiking.

In their recent paper, Dr Carrignon and his colleagues explore this computational model to assess the interplay between post-marital residence rules (i.e. who moves after marriage), transmission pathways (i.e. who we learn from) and frequency (i.e. when we learn), and the demographic consequences of the beneficial innovation on the diffusion cultural practices. Their model highlights the importance of factors such as "re-socialisation" —the act of (re)learning cultural traditions from people in your new community— which may hinder cultural hitchhiking but also lead to more heterogenous communities depending on whether the tradition is 'oblique' (learning from non-parents of the previous generation) or 'horizontal' (learning from others of the same generation). The model also highlights the importance of quantifying how more complex beneficial traits, such as farming economy, can be easily transmitted between communities or the extent of reproductive advantages these traits can confer. A population may grow way faster due to a very efficient subsistence strategy, promoting the hitchhiking of cultural practices unrelated to reproductive advantage. However, if such a new subsistence strategy can be easily copied by other populations, then their cultural practices can be more resilient, counterbalancing the hitchhiking effect.