Present Participial Phrase Modifier

[cyprus]

Dear Instructors,

The below is a sentence from Official Guide 2015 SC (one of the wrong answer choices)

At some point in the past our ancestors suffered an event, greatly reducing their numbers.

OG explains that the agent to the participial phrase is subject of the preceding clause - 'our ancestors' - hence the modifier is incorrectly used.

While I understand that a present participial phrase such as this one can modify the entire preceding clause and specifically applies to/derives from the subject, there seems to be another usage of such a participial phrase that I picked from MGMAT SC book, ie, to hold the result of the preceding clause, such as in the example below, in which the agent of the participial phrase need not necessarily be the subject of the preceding clause:

Crime has recently decreased in our neighborhood, leading to a rise in property prices.

Similarly, I thought that there was no mistake in the modifier of the original sentence, ie, the ancestors' suffering of the event greatly reduced their numbers. Can you please point out if my understanding is wrong here?

Also, have I identified the 2 significantly used use cases for the COMMA + VERBing modifiers as above? Are there any other use cases tested frequently on the GMAT one must be aware of?

Thanks very much for your responses!!

Kind regards

[RonPurewal]

i'm not going to address the grammar terms, because i don't know them.

analogy:
I dropped the bags onto the floor, scaring the dogs.
––> i didn't directly scare the dogs. rather, my dropping the bags scared the dogs.

here's the key question: if you had to assign responsibility for the __ing to someone/something, then to what/whom would you assign it?
that should be the subject.

in the example above, the responsibility would be most directly placed on me, so "I" is the subject.

in your 2nd example, if you have to assign responsibility/causality to a THING, then that thing is ... crime, which caused the rise in property values by decreasing.

[RonPurewal]

keep in mind: we are NOT saying "crime caused property values to rise". we're saying that the entire action described (= crime went down) is responsible.
(similarly, i didn't scare the dogs, although my actions did.)

this is actually the whole point of these modifiers: they are specifically appropriate when it would be incorrect/inappropriate to directly state "X caused Y", but when the blame/causality/responsibility can be assigned to what X did.

[cyprus]

keep in mind: we are NOT saying "crime caused property values to rise". we're saying that the entire action described (= crime went down) is responsible.
(similarly, i didn't scare the dogs, although my actions did.)

this is actually the whole point of these modifiers: they are specifically appropriate when it would be incorrect/inappropriate to directly state "X caused Y", but when the blame/causality/responsibility can be assigned to what X did.

Thanks, Ron, for your explanation.

So in the original sentence below:

At some time in the past the ancestors suffered an event, greatly reducing their numbers.

We ask: who is responsible for greatly reducing the ancestors' numbers? - should be the event and not the ancestors (and thereby as the subject). Hence, you think that the sentence as it stands is wrong - do I understand you correctly?

Kind regards

[RonPurewal]

yes.