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A child is born
"Why do so many Japanese women have given names ending in -ko?" is a question that... actually you don't hear asked that much any more, because names ending in -ko are out of fashion. But for a good few decades in the middle of the 20th century, such names were very popular, to the extent that in any given class of schoolchildren you would expect more female students with names ending in -ko than without.
Now, this wasn't always the case. A century before that, barely anyone had names ending in -ko. So, even from a historical perspective, "What happened?" remains a valid question.
"Ko" no tsuku namae no tanjō 「子」のつく名前の誕生 ("The birth of names ending in ko"), by Hashimoto Junji 橋本淳治 and Itō Nobuhiko 井藤伸比古 is the record of an amateur primary-source research attempt to answer this question (guided by non-amateur Itakura Kiyonobu 板倉聖宣). I'll tell you up front that they don't actually identify a smoking gun, but it was an enjoyable read nonetheless.
Probably the most intriguing part of the story is the sheer fluidity of names well into post-Meiji Restoration times. -Ko was originally a suffix applied out of respect, originally reserved mainly for the nobility. This kind of thinking apparently persisted into the Showa period at least; the authors share several anecdotes about people from older generations who used to call their wives X-ko even though their "real" names were actually just X, books on polite correspondence advising the addition of -ko, and so on. But at the same time, this overlapped with a slightly different understanding of -ko that emerged later: that it was part of the name, not an appended honorific.
Obviously, it was the latter that led to -ko names qua names becoming overwhelmingly popular among the general population. But it's hard to see how this could happen if the understanding of -ko as an honorific was similar to the understanding of, say, -san today. So either the general population was completely ignorant about the use of -ko among the nobility, and reanalyzed it as part of the name rather than as an honorific, or the very distinction between name and honorific was less clear back then.
Now, it wasn't that the common folk had no honorifics at all — they used the O-X-san pattern. So it seems unlikely to me that they just didn't understand what the noble -ko was doing. Which makes the second option more likely, in my opinion.
One interesting fact the authors dig up is that if you look at the names of female characters in popular fiction, there is a distinct boom in -ko names in 1899, the year after the monster hit novel Hototogisu ("The Cuckoo") introduced its heroine Namiko to the world. The percentage of fictional female names ending in -ko jumped from 5% in 1898 to almost 10% the following year, and steadily increased after that until it was at nearly 30% by 1910. The increase in real-life names ending in -ko apparently began around the turn of the century.
The authors also find two essays published around the same time saying, in effect, "Why shouldn't the common people name their daughters X-ko? I think it's lovely." In other words, mass media may well have played the decisive role in promulgating -ko names among the people of Japan, by including them both explicitly and implicitly in the the post-Meiji national identity that the media shaped.
Ali:
That's interesting. I'd noticed there was an excess of -ko names in Western film & tv, but I'd thought it just a terrible cliche that was part of the cookie-cutter Japanese characters, and not something more. Certainly plausible that the wider reach of media played a substantial part in the influence, when you consider in the West, the popularity of actors or their roles often spawned (and still do) trends in baby names.