Noting a correlation is acceptable. But that's not what you did: you posited that Jews support Israel because they are Jewish, and, indeed, that above all other reasons. The generalising logic is quite the same as that which lead to the assumption that all Japanese-Americans were sympathetic to the Empire.
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Not quite the same in my opinion. Japanese-Americans were American and Japanese.
American jews, or Russian Jews, are not Arabs at all. So for them to take sides between Israel and Palestine is not the same issue than for American Japenese to take side between the US and Japan.
I also tend to think that more American Japenese were against the US war against Japan that the US war against Germany. the proportions being the opposite within "German American". I don't buy the "Japenese American" are Japanese and shouldn't be trusted theory. But to deny that some of their thinking would be influenced by their japanese roots is utopia.
In Ayn Rand case, the antagonism between the state of Israel "politics" (socialism) and hers should have inclined her to not support that state, or at least to not care. But she did, I don't see any other reason but her cultural jewishness. I may be wrong, but that option does not seem to be completly irrational.
If you think that Arabs are uniformly pro-Palestine, and this only because of their related ethnic backgrounds, then you don't really really know very much about the Arabs or about Palestine.
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I'm tunisian, french tunisian to be more precise

. I may not know about what all Arabs feel about Palestine and why, but I think I am not completly unknowlageable

I think Arab are by a vast majority pro-Palestine indeed, all pols and "manifestations" tend to show that even if pols in non democratic arab countries are not easy to do. And when I say they support Palestine because of their "ethnic backgroud" that does not mean that it wasn't also "thought". It works like religion: most of muslims are muslims because they were born in a muslim family. They also thought about it and are consiously aware about what they think. But fact is they did not make a comparative study between Islam and other religions and chose Islam, I even think that iff they do, the majority would still stick to Islam. The same reasoning applies to other religions by the ways as 99% of christians are of christian heritage and so are Buddist and Hinduist. I therefore tend to think that the Arabs vast support for Palestine is mainly due to ethnic reason. They do also find the "cause" to be just. But very often that is the consequence. The very fact of "caring" about the problem itself is because of ethnic reasons: why should a Tunisian care about what is happening in Palestine and not about what is happening in Timor?! or Northern Ireland some years ago? or even South Africa Apartheid?
And, I'll point out, I didn't say that you are anti-Semitic, or that you are being anti-Semitic, just that what you said sounded anti-Semitic, with the implication that this should lead you to inspect it more closely. That's a crucial and sincere distinction.
So because some Jews came to that conclusion, all individuals of Jewish ancestry who supported Israel can simply be assumed to support that conclusion? That's pure conjecture.
Not all, I never said all jews support Israel because they are jewish because first not all jews supoprt Israel, and many do not because they are jewish but because of other reasons. Now what I am saying is:
1. you'll find more support for Israel among jews than among "buddist" because being jewish make you closer to the "jewish" cause, as being of armenian ancestry would make more sympathetic to Armenia. All that being on average and not specifically for every person.
2. therefore, since Ayn Rand political views tended to be against those of the early Israel, and that she nevertheless supported that State, thinking that such support is due to her jewishness does not seem to me as being "irrational" or leaning towards anti-semitism.