In Japan by 2531 they could all have the same surname, “Sato”, here's why

All inhabitants of Japan with the same surname in about 500 years? It seems impossible, but due to a Japanese law that requires married couples to adopt the same surname, it could really happen, at least mathematically speaking. In fact, every time a marriage is contracted in Japan, one of the two surnames is “lost”: this can cause a rapid “extinction” of the less common surnames and, on the contrary, an increase in the relevance of the more common ones, up to the survival of a single surname, which according to a study conducted by Hiroshi Yoshida of Tohoku University would be Sato.

Increase in the dominant surname and the risk of losing some surnames

In Japan, at the time of marriage, the law requires that a the only family surname for the couple. The surname can be that of the wife or husband indifferently, even if in 95.5% in some cases the surname chosen is that of the husband. This, together with other factors, leads to a loss of some surnames from year to year and to thestatement by others, which increase their percentage presence in the population. This is the case of the surname Sato.

The Tohoky University study states that if we call ρ the growth rate of the surname Sato e x(t) the percentage of individuals with the surname Sato in the year tbetween 2022 and 2023 there was a growth factor equal to (1+ρ) = 1.0083:

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x(2023) = (1 + ρ) · surname(2022)

This means, in simple words, that by projecting this growth into the future and reiterating the above calculation, we obtain that in 2531 the percentage of individuals with a surname Satowhich in 2023 was equal to 1.53% of the Japanese population, will reach 100%.

Remember that this is a prediction based on a mathematical model, but there are many contingent factors that can influence it growth or decrease of a surname (in our case Sato):

• The marriage, which can make the surname increase or decrease depending on whether or not it is chosen as the family surname;
• The divorce, which can decrease when it had been chosen as the family surname and is “lost” by one of the two spouses, or increase in the case in which it had not been chosen as the family surname and is “taken back” by one of the two spouses;
• there birth of a son within a family with the surname Sato, which increases the percentage of the surname;
• there death of an individual surname Sato, which decreases the surname;
• The demographic declinewhich makes uncommon surnames “disappear” and on the contrary favors the proportional increase of the most common ones;
• other factors, such as increases due to voluntary change of surname or to the naturalization of foreigners.

What happens if surname laws change?

The drastic one reduction of variety of surnames in Japan is strongly influenced by the fact that today – at the time of marriage – one of the two surnames must “disappear”. Professor Yoshida then analyzed what would happen if there was a change of scenario and the laws changed so as to allow each couple to choose whether to opt for a unique family surname or both keep their surname of origin.

The study was based on the results of the survey “What would you do if selective separate surnames were introduced for married couples?” conducted by Japan Federation of Trade Unions. According to this survey, the growth factor of the surname Sato is reduced to 1.00325. Repeating the calculations made previously with this new value, it is expected that the surname Sato it will still achieve hegemonybut will do so in the year 3310.

Could it happen in Italy too?

Currently, in Italy, there are around 59 million inhabitantswhile I surnames I am alone” 350,000 about. This means that each surname is carried on average by 170 peoplebut there are surnames that are not very common and others that are very widespread: Rossi, for example, it almost counts 80,000 families in Italy.

However, in our country we have a great one linguistic differentiation due to dialectsfor which many surnames have undergone over time variations, thus generating over time new surnames. This great linguistic variety is however strongly threatened by the fact that in Italy, as in Japan, we are hit by a demographic decline. This fact, in addition to aging the average age of the Italian population, threatens the variety of surnames which is historically very flourishing in Italy.

However, something is certainly changing: until 2022 the father's surname was automatically attributed to children, thus leading to a possible disappearance of the maternal surname. From the June 2, 2022 the law has been changed and newborns automatically assume a double surnamethat is, the union of the parents' surnames.

The estimate more than a year after this change of scenario is that iThe double surname was assumed by 5 to 18% of newborns to a variable extent from Region to Region, and this the probability of survival of one in five maternal surnames increases.

In short, we can say that the situation in Italy is quite different from that of Japan, so at least for the next 1500 years we won't all be called Rossi. Unless the laws change!