I noted that giving a downvote to an answer deducts $-1$ from one's own reputation.

But this means, downvoting can be identified by a change from odd to even reputation $-1$ (and vice-versa)?

For example, if one notes a new answer by some user and simultaneously his reputation changes $-1$, one can almost surely infer it was this user who downvoted. The downvoted user might go to the downvoter's account and randomly downvote some of his posts just in a way that avoids being detected as serial downvoting by the SE algorithm to avenge himself.

I consider this as a bug which should be repaired, since voting should not be public in general.

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    $\begingroup$ Yes, if one is smart enough, one can sometimes figure out who was behind the downvote. Just like if one is smart enough, one can figure out who killed JFK, whether or not the NSA has brain-controlling viruses in our food, and if and when the KGB will return to power and take over the world. $\endgroup$ – Asaf Karagila Jun 25 '14 at 19:04
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    $\begingroup$ Also, downvoting questions costs nothing. $\endgroup$ – Asaf Karagila Jun 25 '14 at 19:05
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    $\begingroup$ I think you dont understand the point that the downvote can be detected, which it shouldnt. This is a technical issue not a hypothetic one. $\endgroup$ – emcor Jun 25 '14 at 19:07
  • $\begingroup$ Yes, downvotes can be detected. Upvotes too sometimes. You just need to be very observant. $\endgroup$ – Asaf Karagila Jun 25 '14 at 19:09
  • $\begingroup$ But you cannot know who gave the Upvote. Downvote is unique by a change in $-1$ at the Downvoter. $\endgroup$ – emcor Jun 25 '14 at 19:11
  • $\begingroup$ Unless you note in details the reputation of the user, it's hard to tell whether or not they have been downvoting or not. Not to mention, that sometimes a user can be active on several threads. Not to mention that downvotes to questions carry no penalty. $\endgroup$ – Asaf Karagila Jun 25 '14 at 19:12
  • $\begingroup$ Do you understand that you are wrong? Do you understand that one gets $15$ points when their answer is accepted, which turns odd reputation to even reputation without voting? Do you understand that voting on a question also gives the receiver $5$ points which again changes the parity of the reputation? Do you understand how the reputation system works at all? $\endgroup$ – Asaf Karagila Jun 25 '14 at 19:19
  • $\begingroup$ Ok that is a fair comment, which is also the reason why I asked this -question-. But still when you observe a downvote and reputation -1 simultaineously, it should not be uniquely attributable to this user having downvoted? $\endgroup$ – emcor Jun 25 '14 at 19:22
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    $\begingroup$ Yes, it can be. But can you be sure that this downvote was to your answer? (Because again, a downvote to a question costs nothing.) $\endgroup$ – Asaf Karagila Jun 25 '14 at 19:43
  • $\begingroup$ This might be fixed by randomly delaying the time of deduction, or just setting the deduction to -2 instead, which is then less identifiable. $\endgroup$ – emcor Jun 25 '14 at 19:48
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    $\begingroup$ @AsafKaragila: Anyone with half a brain knows Elvis killed JFK, funded by Big Tobacco. $\endgroup$ – gnometorule Jun 25 '14 at 20:21
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    $\begingroup$ @gnometorule: How about people with an entire brain. What do they know about that? :-) $\endgroup$ – Asaf Karagila Jun 26 '14 at 19:32
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    $\begingroup$ @AsafKaragila: everyone who has half a brain has in particular an entire brain. $\endgroup$ – user159761 Jun 26 '14 at 22:22
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    $\begingroup$ Why downvote this excellent question? That things are more complicated than what the OP thought proves that he was right to ask. The enthiusiastic upvotes of the answers confirm this. I wonder who so unjudiciously downvoted the question... $\endgroup$ – Georges Elencwajg Jun 27 '14 at 10:01
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    $\begingroup$ @GeorgesElencwajg: It may be that down votes reflect a disagreement (this being meta) with a perceived suggestion in the post (rather than asking a good question) to prevent the possibility of downvoting Answers being attributed. $\endgroup$ – hardmath Jun 27 '14 at 16:35

Yes, if one suspects that a particular user has downvoted one's answer, then tracking the reputation of the suspect can confirm the suspicion. This has been described on meta already:

Open the page of the user X in question and record his reputation. Delete your downvoted answer. Wait a few minutes so that the system implements the reputation changes (in particular, your own reputation should decrease by appropriate amount) and then refresh the page of X. If he was at the origin of the downvote, his reputation will be increased by 1. Then undelete your answer, wait again and refresh the page of X. His reputation will decrease back by 1.

The system already delays the reputation recalc somewhat. Setting the deduction to $-2$ or another number would not change much: users can still see the expected changes of reputation upon deleting/undeleting the answer (and they can do several rounds of delete/undelete, choosing time when the site is not very active and the suspected user is not online).

So: yes, downvotes on answers are not $100\%$ anonymous. This is widely known: see Anonymity of downvotes has a loophole where a Stack Exchange employee commented:

Yes, it is a loophole of sorts [...] this does bother me a bit. A dedicated user with a vendetta could confirm his target much more readily now than in the past. We'll want to keep an eye on it.

That said, there is an easy solution to the problem of revenge downvotes, serial downvotes, etc. It's something I call "not giving a damn".

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    $\begingroup$ It would not make sense to assume "not giving a damn" because the downvoting feature exists for it does matter. $\endgroup$ – emcor Jun 25 '14 at 20:08
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    $\begingroup$ @emcor Voting, up or down, is a way to put some number next to each post. That number is then used by the software for various things like sorting or filtering. There is no need for the author of the post to lose sleep over the value of that number. $\endgroup$ – user147263 Jun 25 '14 at 20:46
  • $\begingroup$ I see your point in that one must not take the "reputation score" too personal in a way. One may argue that it is but linked to badges and other privileges for the user. I accept your answer though. $\endgroup$ – emcor Jun 25 '14 at 20:56
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    $\begingroup$ "There is no need for the author of the post to lose sleep over the value of that number" Yeah, that dude François de La Rochefoucauld said it well: Nous avons tous assez de force pour supporter les maux d'autrui (Maxime 19) $\endgroup$ – Georges Elencwajg Jun 27 '14 at 10:13

For the moment, this is by design.

It honestly hasn't been a problem to date and when someone is able to accurately figure out who downvoted them, it's usually beneficial - for example, for identifying revenge downvoting. In most cases, however, folks just make (frequently wrong) assumptions rather than spend time and effort to correlate timestamps.

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    $\begingroup$ Revenge downvoting occurs from identifying downvoting, so its actually a consequence of this problem. If identifying downvoting was beneficial, it would be allowed and not treated anonymous as it is. $\endgroup$ – emcor Jun 26 '14 at 19:45
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    $\begingroup$ @emcor Nah, revenge downvoting occurs in a number of different scenarios. "I don't like what this guy said to me" seems to be the most common in my experience. $\endgroup$ – Adam Lear Jun 26 '14 at 19:46
  • $\begingroup$ What benefit do you have from identifying revenge downvoting? $\endgroup$ – emcor Jun 26 '14 at 19:48
  • $\begingroup$ We try to keep the playing field fair. So going and targeting another user with votes (either up or down) is a practice we discourage and/or punish as needed. $\endgroup$ – Adam Lear Jun 26 '14 at 19:50
  • $\begingroup$ That is from standpoint of SE supervisors not users, SE has an algorithm to detect serial down/upvoting each day. Why do you think downvoting is anonymous if it was beneficial to identify? $\endgroup$ – emcor Jun 26 '14 at 19:53
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    $\begingroup$ I'm not saying that users identifying downvotes is a key part of anything. Nobody is relying on it. Heck, pretty much nobody actually does it. We occasionally get an "I seem to be getting a lot of downvotes" email, but even those very rarely identify the potential culprit. All I'm saying is that if someone is able to accurately guess, it can have an upside. This isn't a problem in practice. $\endgroup$ – Adam Lear Jun 26 '14 at 19:59
  • $\begingroup$ I find it strange how you think nobody does it, when its described in the answer post above, and nobody who does it would actually hint that he did it. I neither see identifying a potential revenge downvote as an upside, because you cant do anything about it then unless its serial downvoting. $\endgroup$ – emcor Jun 26 '14 at 20:21
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    $\begingroup$ I may be underestimating how often this happens, sure. (And on the flip side, you may be overestimating. :)) But unless there are practical implications, this still seems like a non-issue. $\endgroup$ – Adam Lear Jun 26 '14 at 20:22
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    $\begingroup$ Ok, I would agree to that. When I was a new user, I was quite curious about who may have downvoted me because reputation counts so much to get the basic privileges. $\endgroup$ – emcor Jun 26 '14 at 20:26

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