Let's remember that they're aren't any fewer Catholic today than there ever were. There's just fewer Catholics at Mass on Sunday, in the convents, in the seminaries, in Catholic schools, etc. But the population of Catholics is larger than ever. And nearly all of them go through the ritual (which they probably consider no more religious than putting up a Christmas tree) of having their children baptized.
the number of marriages has plummeted since 1990 by around 30%
Actually it's a lot worse than that. Compare the peak year of 1970 with 426,309 marriages compared to 2002 with 241,727. That's a drop of 43% or nearly half. So with statistically more self-identified Catholics in the US, we have just more than half as many marriages.
Meanwhile look at the number of baptisms which peaked in 1961 at 1,352,371 and then dropped to 876,306 in 1974 before rebounding to 1,005,490 in 2002. Compared to 1974 it's true that this does represent an outright increase in baptisms. But it does not mean that Catholics are having more kids. There are a lot more "Catholics" in the United States today than there were in 1961, even if many of them are co-habitating instead of getting married. (Does your source book for this data have a number for total Catholics?) So the birth rate is still much, much lower.
Turning the drop in marriages into a good thing by using it as the denominator in the equation does not make sense anymore. It was a valid comparison for as long as the percentage of illegitimacy was so low as to be negligible. That would apply until 1964 or so. But today you're comparing apples and oranges.
Do you have evidence for this? The suggestion makes me curious enough that I'm going to ask some Priest friends about it. Maybe Deacon Sinky or TM can tell us from their ministerial experience if they get many single mom's for Baptisms.
the number of marriages has plummeted since 1990 by around 30%
Actually it's a lot worse than that. Compare the peak year of 1970 with 426,309 marriages compared to 2002 with 241,727. That's a drop of 43% or nearly half. So with statistically more self-identified Catholics in the US, we have just more than half as many marriages.
The marriages come out of the baptisms/births of 25 years previous, in general, so the tracking is pretty good of the trends. The present dearth of marriages is due to the so-called "Birth Dearth" of 1968-1988. That the number of baptisms is holding steady as Marriages plummet is a good thing family-size wise. It was precisely those people married in the peak wedding years of 1968 through 1973 who had the fewest children.
What is remarkable is that the upwards bubble of Baptisms from 1952 to 1964 did not produce the slightest blip in Marriages from 1977 to 1989 - they held steady at 350,000 per year, even as the number of Catholic theoretically eligible increased by 30%. It is clear a large portion of that cohort has left the Church.
Meanwhile look at the number of baptisms which peaked in 1961 at 1,352,371 and then dropped to 876,306 in 1974 before rebounding to 1,005,490 in 2002. Compared to 1974 it's true that this does represent an outright increase in baptisms. But it does not mean that Catholics are having more kids. There are a lot more "Catholics" in the United States today than there were in 1961, even if many of them are co-habitating instead of getting married.
I disagree here, because I do not see major changes in illegitimacy behavior between 1983, 1993, and 2003. The 1990's were noted for a slowing in illegitimacy, not a speed up. Yes, the number of Catholics is up, but that is because births plus immigration plus conversion have outnumbered deaths plus apostacies. It is like society though, in that Baptisms are holding steady even as the number of Catholics of child-bearing age is clearly dwindling, since it is now the "Birth-Dearth's" turn.
(Does your source book for this data have a number for total Catholics?) So the birth rate is still much, much lower.
It has the total registered, which now stands at around 62 million or so. I did a calculation of apostasies in the Church before from 1965 to 1993, and came up with the number of 8.5 million, including some funny jumps in the total number of Catholics in a few years. The calculation was annual change in Total Catholics - Baptisms - Converts + Deaths. This has significantly slowed the growth of the Church in the US since Vatican II.
On the other hand, the number of converts is up markedy in the past ten years, to an annual level of 160-180,000 in the US for at least the past five years, the highest ever, and a doubling of the numeric rate of 20-30 years ago.