Now I am reading A Renegade History of the United States by Thaddeus Russell, history Phd, in which he cites post-slavery surveys of slaves that determined that only between 1% and 2% of slave-owners would rape slaves. He also gives stats on the number of mullato slave children as an indicator to how rare it was.

This is absolute bullshit.

It is hard to put an exact number on it, as not every master or overseer was going to be keeping track (although check out /u/TRB1783's answer for a good look at the culture srrounding it), but rape (and I use rape here because no matter how willing to go along with the act a given slave might have been, the power dynamics of the relationship should unnerve anyone, so lets not get into a semantic debate) of black slave women by whites was very common, and we can trace this fact through genetic testing.

I did a brief explanation of this previously, which I'll copy paste for you here:

Anyways, the rise of cheap, easy DNA testing has been a real boon for people looking into their roots, and also has given us some very interesting peeks into history. A little while back, TheRoot.com published a piece summarizing the findings of mass-market DNA testing companies from when they analyzed African-American DNA.

What did they find? Depending on the company, the average percentage of DNA in African-Americans that was of European ancestry was between 19 percent and 29 percent. For the patrilineal line specifically (father's father's father's etc), 35 percent of African-Americans would eventually hit a white ancestor. And of course that doesn't account for maternal great-x-grandfathers. If we assume that the white DNA is almost exclusively coming from a male ancestor (not to say it couldn't happen where a white woman slept with a black man, but I think it reasonable to assume it happened less frequently), than we can double those numbers, and say that the average African-American's forefathers are very white. Between 38 and 58 percent in fact. I would think the implications of that are clear, but just to drive home the point, a LOT of that comes from masters or foremen having sex with African-American slaves, possibly in violent circumstances, but almost certainly as part of a coercive relationship where they couldn't exactly say "sorry, I have a headache tonight".

The TL;DR is that the average African-American is actually 1/4 "white".

As for why the survey's Russell cites would indicate such a low percentage given the genetic evidence to the contrary, having not read his book or familiarized myself with the survey I can't give a definitive answer, but I would point out that in the post-war south, the black population was still on the bottom rung, and while now not enslaved, often found themselves working the same fields for practically the same people, just for some poor wage. They had a LOT of incentive not to make waves.

I don't know where Russell would get that figure from: Not all slave owners would write about sex with their slaves, and slaves certainly had no legal recourse for prosecuting their masters for rape. As such, it would be impossible to come up with any kind of exact percentage of the frequency of slave rape.

To answer a question that has popped up in this thread already, it can be unclear how exactly rape would be defined in this case. Certainly, female slaves had no real way to refuse their masters' advances. As such, it would be pretty difficult to give consent in the modern definition of the word. It is worth noting that in cases where one party is incapable of giving legal consent in modern society, such as minors having sex with adults or prisoners and prison guards, all sexual contact is considered rape. One could make a case that this standard be applied to sex between masters and slaves, too.

In any case, the 1-2% figure of absurd. There was from the earliest period a large and visible mixed-race population of enslaved people. Some particularly honest diarists, like William Byrd Carter, gleefully reported their assaults on their slaves. There is no instance I can think of a slave owner facing any kind of social disgrace for his conduct towards his female slaves.

Walter Johnson, in River of Dark Dreams, goes a step further, stating that the sexual exploitation of female slaves was a way for slaveowners to increase the number of slaves they owned naturally. We know Jefferson kept his own children enslaved, and he was hardly the only Southerner to do so.

As for Russell and his "Renegade History..." well, if this is the level of history he writes, it's no wonder he got kicked out of academia. People don't get fired for telling "unpopular" histories; they get fired for being bad historians. Ignoring the overwhelming weight of primary and secondary sources that detail the sexual exploitation of enslaved people is a great and highly visible way to proclaim to the world that you're a crappy historian. My guess is that he's decided there's (STILL) a market that will gobble up "evidence" that the Slave South/Confederacy "wasn't so bad" and has chosen to pander to that set. The fact that he'll make more money off book sales than Eric Foner is REALLY frustrating.

EDIT:

He even claims that there were instances of slaves assaulting slave-masters with no repercussions.

I can think of one instance of this happening: an overseer was hitting a leased/rented slave, and the slave knocked him down, took his whip, and beat the overseer. The slave was not punished, as the overseer technically had no right to damage the actual slaveowner's property. It had nothing to do about any inherent rights or dignity the slave himself possessed. I think the story is told in Walter Johnson's Soul by Soul.

Can i ask a follow up question.

Could you actually rape a slave, as in was it termed rape or was it considered legal to do so?

Basically the indirect consequences for raping a slave would massively deter doing so.

Does he explain what he means by this? Does he mean that injuring a female slave through a violent rape would limit her productivity in the field? Does he support this by any evidence at all, or is this just an unbacked idea that he has?

I would encourage you, OP, when reading history books or listening to people assert historical arguments to consider their evidence and their point of view. Because, in the end, you can believe Thaddeus Russell, or your college professor, or another book about slavery, or you can form your own opinion based on what research you've done. But you have to know where the evidence lies and what problems there are with that evidence.

You should always try to find out just what a historian is basing his argument on. Just what is the evidence when we discuss slavery? What biases does that evidence have? If we're talking about the harshness of slavery, can we trust plantation owners, or overseers, or abolitionists, or ex-slaves in the 1860s, or ex-slaves in the 1920s? You should do your best to form your own argument based on what you find, especially on controversial arguments. On sensitive and provocative topics such as slavery, historians and laypeople alike will want to make historical assertions, and they may or may not have any evidence at all to back up their thoughts. But they sure as hell will fight desperately to defend their ideas.

I would be interested to read the survey. Do you have a source for the study that he cited? Or is there more follow up discussion? It would be informative to know things like, how many slaves were surveyed, who surveyed them, where they were surveyed (North or South), why they were surveyed and if the survey was self reported?