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In this text, why do we use the past-perfect here instead of the simple past tense for "double"?
The chart shows the percentage of people aged 65 and over in the United States between 1900 and 2000. In the year 1900 just over 4% of the population was aged over 65. However, by 1960 this figure had doubled.
(emphasis added)
I've suggested this reference when similar questions come up. Here's an excerpt explaining the two cases it is used:
The Past Perfect expresses the idea that something occurred before another action in the past. It can also show that something happened before a specific time in the past.
With Non-Continuous Verbs and some non-continuous uses of Mixed Verbs, we use the Past Perfect to show that something started in the past and continued up until another action in the past.
This is a bit tricky, but the way I'm understanding it, this sentence:
The chart shows the percentage of people aged 65 and over in the United States between 1900 and 2000.
establishes a context where we are talking about the population from 1900 to 2000.
So, the "before" action or specific time that this sentence references
However, by 1960 this figure had doubled.
is the percentage of people aged 65 and over in the United States in 2000 (the end of the implied progressive action of the population growing from 1900 to 2000).
The "before" action or specific time may not always be explicit. In cases like this where the connection is rather weak and the time is explicitly specified I don't believe it's wrong to just use the simple past tense.
However, by 1960 this figure doubled.
This doesn't sound wrong to me.
When you say 'by a point in time', you use the perfect tense. As it is 'by 1960' i.e. a referral to a time in the past, the past perfect should be used. The use of past tense sounds unnatural.
The "by [some point in time]" usually means that whatever you are talking about happened before that point in time.
When talking about the future, e.g.
The possibility that we will have mostly decarbonized our economies by the time nuclear fusion becomes commercially viable is not a given. The New York Times
the future perfect is used precisely because you are talking about the past from the point of view of a future event (in the example, nuclear fusion is in the future and decarbonization is also in the future but in the past in relation to nuclear fusion).
So in most cases using this construction "by + [past event]" you will use the past perfect. In your example, the figure already had doubled by the time 1960 arrives, i.e. even in 1960 the doubling is in the past, and because 1960 is also in the past, we use past perfect.
However, there is a situation where simple past may be used, and that's with state verbs. e.g.
By then, she had different interests and remembered nothing of the period I had written about. The New York Times
because on that past time referred by the "by", the states (having different interests and remembering things of the mentioned period) were still true, so the simple past is in reference to the same time period as the "by", and not a time before.
Why not say: In 1900, just 4% of the population was over 60 years old, but by 1960, that percentage doubled.
If you just use the simple past tense, the sentence will seem "off" to many native speakers.
However, by 1960 this figure doubled.
The confusion isn't really over which tense to use. It could be as you think of it, a simple action that happened in the past. 1960 is obviously the past of now.
Rather, it's subject/object confusion. As written, it's unambiguous:
However, by 1960 this figure had doubled.
When we use the past perfect tense here, most readers assume that the subject (this figure) and the object (this figure) are the same. We could write:
However, by 1960 this figure had doubled itself.
But that isn't really necessary, most native speakers would understand "figure had doubled" to mean that the value of the figure had become twice what it once was.
But when we write,
However, by 1960 this figure doubled.
It can seem incomplete. What has the figure doubled? Had it doubled the number of Americans receiving old age pensions? Had it doubled the number of Americans in nursing homes? Or had it simply doubled itself, the number of Americans over 65?
I realize that it probably seems incomprehensible that "had doubled" resolves this confusion, since we could just as easily write:
However, by 1960, this figure had doubled the number of Americans in nursing homes.
But somehow, it does resolve that confusion. If no other object is specified, it had doubled itself.
And as others have mentioned, since 1960 is a specific point in the past, it is at least technically more correct to use the past perfect tense.
