claim (a) is that there are such differences that contribute (substantively) to different outcomes
That is not claim (a). Claim (a) is that there are genetic factors in group differences, but makes no assertion about their magnitude. You believe that genetic factors make at most a small contribution. You may well be right. But you have offered no evidence for this assertion, and the burden of proof is on you to show it.
I would assume (b) is false without evidence as well.
Given that different groups live in manifestly different physical and social environments, this assumption is also wrong. The null hypothesis is that both (a) and (b) contribute; the burden of proof in both cases is on those who think one or the other is false. Confusing this issue, whether intentionally or unintentionally, is perhaps the most common source of crimestop on this subject. (Not that I blame you; as a crimethinker myself, I can assure you that volunteer Thought Police are everywhere, even—perhaps especially—on HN.)
Of course, in reality the discussion usually goes something like this:
"There might be genetic factors accounting for differences in group outcomes."
"I doubt it, but even if there are such factors, they're small."
"How do you know they're small?"
"Well, how do you know they're not?"
[Caught in trap] "I don't. So let's investigate the magnitude of the effect by examining the direct evidence…"
That is not claim (a). Claim (a) is that there are genetic factors in group differences, but makes no assertion about their magnitude. You believe that genetic factors make at most a small contribution. You may well be right. But you have offered no evidence for this assertion, and the burden of proof is on you to show it.
I would assume (b) is false without evidence as well.
Given that different groups live in manifestly different physical and social environments, this assumption is also wrong. The null hypothesis is that both (a) and (b) contribute; the burden of proof in both cases is on those who think one or the other is false. Confusing this issue, whether intentionally or unintentionally, is perhaps the most common source of crimestop on this subject. (Not that I blame you; as a crimethinker myself, I can assure you that volunteer Thought Police are everywhere, even—perhaps especially—on HN.)
Of course, in reality the discussion usually goes something like this:
"There might be genetic factors accounting for differences in group outcomes."
"I doubt it, but even if there are such factors, they're small."
"How do you know they're small?"
"Well, how do you know they're not?"
[Caught in trap] "I don't. So let's investigate the magnitude of the effect by examining the direct evidence…"
"That's racist."