There will be much more to say – not least, perhaps, with regard to the definition of "genocide" – but an earlier thread here dealt with Nazi policy with regard to the disabled, a group whose members included some people who I suppose meet your definition of "dumb". You might like to review that thread while you wait for fresh responses to your question.

Are the experiences of handicapped people in the Third Reich comparable to other prosecuted groups?, with u/commiespaceinvader and u/UWCG

Can I just address the point you do make in the question as well, the so called 'genocide of intellectuals'. Intellectuals is not a victim category in the UN definition of the crime of genocide. The two categories of victims that the Khmer Rouge can be said to have perpetrated genocide against in Cambodia were the ethnic Vietnamese minority and the Muslim Cham minority (comprising at most 5% of the total death toll). This is the case because they were (as more or less proven by the Khmer Rouge tribunals) targeted on the basis of their religion and ethnicity. The vast majority of deaths that occurred were Cambodians themselves, and they were not killed on the basis of their ethnicity or nationality, but rather that they were broadly 'anti-revolutionary', i.e these were politically motivated killings - the group of people being targetted was on the basis of their 'political group', which is not a category in the UN definition.

A more accurate overall description (despite the period being generally known as 'the Cambodian Genocide') is that it was crimes against humanity and widespread government sponsored mass murder. I guess I should say I do consider myself a 'definitionalist' (a somewhat butchered term but I saw it in a paper discussing this) and do just use the UN definition as opposed to wider, academic, 'widening' of the definition. There is conjecture in the literature about what the definition of genocide should be, and why it is what it currently is, but ... as it is what it is I'm going to use it ... as it is (yikes).

The Khmer Rouge were responsible for around 2 million deaths in the roughly four years they were in power, a higher proportion of those came from the so called 'new people' designation given to those who had been forcibly moved from the urban areas. They were a suspect class in the new social hierarchy and were more likely to be killed under the guise of the March 1976 directive from the Central Committee allowing lower level cadres to 'smash' enemies of the revolution. I will copy and paste the response from a previous answer to just give a better break down of the so called 'anti-intellectual' killings:

So lets run through some wide ideological motivations and goals of the Communist Party of Kampuchea (CPK) and lead into some of the groups targeted by the regime, the reasons for this and the outcomes of targeting specific classes or categories of people had.

No doubt everyone is aware of the forced evacuations of all major cities and towns once the civil war was over. It is important to remember that the civil war was brutal and lasted more than five years. The Khmer Rouge, upon winning, were sure to repress all possibilities of revolt or armed resistance. This goes a long way to explaining the decision to execute large sections of the former government and military who had surrendered in April 1975. Previously they had claimed that only the highest officials of the Khmer Republic regime (Lon Nol, Sirik Matak etc) would be killed but this in practise turned out to be widespread reprisal killings. Not unheard of in revolutionary wars or civil wars.

The net was widened to include anyone who had close ties to the government or military. People’s biographical details were collected in the village co-operatives or on the journey into the country side and those with sufficiently ‘suspect’ roles would often be killed. These early points, or the at least the forced evacuations, are thought to have resulted in around 30,000 deaths. Although that figure also includes anyone who died indirectly from the evacuation not simply executed. These first waves of killings, as well as the forced evacuations, were all directed at smashing any kind of resistance or dispersing any suspected groups from co-operating against the new regime. The secondary point of this policy is the ideological goals of forming co-operatives, collectivising labour and producing a true communist state. In order to do this in ‘record time’, the regime completely levelled the nation and inverted the social hierarchy to idolise the peasant and demonise all aspects of life in the previous regime considered ‘capitalist’ or ‘bourgeois’. This is important because it emphasises the centrality of every single person achieving a kind of ‘pure’ state of mind modelled on an idealised new person – a ‘Kampuchean’ – who was essentially ego-less, focused on serving the party above all else and who would work toward building the revolution.

The CPK divided the nation into new categories based on their class background and their perceived ability to cultivate this ‘revolutionary consciousness’ (khmer: Saitarama). So ‘Base people’, the former peasant class which amounted to more than half of the total population (about 4 million) were afforded more rights in the new social hierarchy of DK. They had supported the revolution longer and were untainted by the capitalist and foreign attributes associated with the other new category ‘New People’.

This is where we can start talking about the targeting of intellectuals.

So, as is probably known to you and most people who are familiar with the subject, it could take only the smallest infraction to raise the ire of a local Khmer Rouge cadre or soldier who had overall goal of ‘smashing enemies of the revolution’, with the cultivation of ‘revolutionary consciousness’ being the guide line for deciding who was an enemy or not. So, for instance – a new person, a young woman, who was caught stealing an ear of corn which had fallen onto the ground, this could be seen as a demonstration of non-collective thinking, selfishness and stealing from the revolution. This is ‘non-revolutionary consciousness’, indicative of someone who was an enemy of the regime. You could be killed for such an infraction – or they could be spared. There were no ‘rules’ for this kind of conduct. We can think about someone wearing glasses being targeted in a similar way. They would be a new person, as the base class didn’t have access to eyewear, and if they were perceived to have stepped out of line, they were more likely to be killed.

There was no directive from the CPK that told their cadre to kill those who wore glasses.

That kind of initiative is more of a ‘bottom up’ process where individual cadre were killing those suspected of harbouring a regressive ‘non-proletarian consciousness’. Wearing glasses is a distinct link to a former class category. I think it is more useful to think of this targeting under the guise of ‘class struggle’ in Marxist-Leninist terms. Wearing glasses was a feature of the urban class – a class which had a higher percentage of deaths in DK. That is the ideological view of this targeting, but it can also be seen as a more simple localised and individual action of a cadre – mostly young and uneducated – making their decision based on this ‘overt’ sign of their former class background, rather than ‘lets kill all the intellectuals’, its more… lets uncover and smash anyone who is unwilling to support this revolution. That kind of rhetoric can be interpreted in different ways and people suffered in disproportionate ways in different Zones in the regime, some were more harsh than others.

So how do we explain the higher numbers of urban khmer deaths in DK? It is not as simple as saying they killed all the intellectuals. The whole category was ‘suspect’, and they were more likely to be killed for any given infraction. They were also more likely to die from overwork and malnutrition due to the foreigness of this new extremely harsh lifestyle. The base people were more likely to survive these conditions based on their previous lifestyle. The attainment of this idealised version of ‘the kampuchean’ was the imperative – but someone’s class background could hinder or perhaps make that attainment impossible. Therefore the ideology of the Khmer Rouge, as well as their willingness to execute people rather than ‘re-educate’ led to a high number of the former urban class being murdered. There certainly are cases where witnesses recalled people with glasses being singled out and executed probably on that basis alone, but this all had to do with class background, which isn’t too dissimilar from saying they ‘targeted intellectuals’. They certainly did. But it was because of what ‘being an intellectual’ meant in regards to cultivating the correct ‘revolutionary consciousness’.

I realise this is kind of a wishy-washy way of basically agreeing/confirming your answer and the overall sentiment, the glasses thing is a cliché for a reason, people with glasses were definitely worse off during DK, and it certainly encapsulates a lot about the nightmare of the regime’s time in power in a succinct detail, but it has become such a staple of discussion about the Khmer Rouge that I thought I would explore it in a little more detail. It should be noted that the Issarak (the antecedent to the Khmer Rouge in the 50’s) also killed people who wore glasses and I believe the Viet Minh were guilty of this too in North Vietnam (I’d have to check that though).

So onto the outcomes of targeting the urban class and the suppression/targeting of ‘intellectuals’.

A Central Committee (the inner circle of the CPK) directive warned in 1976;

We must heighten our revolutionary vigilance towards professors, doctors, engineers and other technical personnel. The policy of our Party is not to employ them … otherwise they will infiltrate our ranks each year more deeply … for the workers of the old regime, we also do not employ them any longer unless we know their background quite well.

The classic American quote here is "Three generations of imbeciles are enough.", from Justice Oliver Wendell Holmes, Jr, in Carrie Buck v. John Hendren Bell, Superintendent of State Colony for Epileptics and Feeble Minded.

At issue in the case was whether the state could, with due process, sterilize someone against their will. This arose from the Virginia Sterilization Act of 1924 (wikipedia link, but includes full statutory text), legislation meant to compel the sterilization of anyone whom the superintendent of a state mental institution feels "that it is for the best interests of the patients and of society that any inmate of the institution under his care should be sexually sterilized, such superintendent is hereby authorized to perform, or cause to be performed by some capable physicians or surgeon, the operation of sterilization on any such patient confined in such institution afflicted with hereditary forms of insanity that are recurrent, idiocy, imbecility, feeble-mindedness or epilepsy; provided that such superintendent shall have first complied with the requirements of this act."

Due process, in this case, was petitioning a special board of directors of the hospital - which means the entire process was a sham. This is why Buck v. Bell is considered next to Dred Scott, Korematsu, and Plessy v. Ferguson as some of the worst decisions in SCOTUS history.

In theory, eugenics acts were race neutral. In reality, of course, rich people could ensure their family never made it in front of one of those boards (or the inverse - Rosemary Kennedy was lobotomized basically on her father's orders), and the sterilization was commonly applied more to women than men, non-white than whites. Here's a well-sourced article that covers the basics and links to deeper dive material. Note that current estimates are north of 60,000 people sterilized from just these programs.

I want to take a moment and point out that by design, eugenics based on 'intelligence' was always going to be racist. The original IQ test was designed to identify French children with intellectual disabilities to provide them help, and the instant Henry Goddard (an American eugenicist) got hold of it, it was immediately put to use categorizing people who might need help whoever Henry Goddard already disliked.

Known as the father of intelligence testing in the United States, [Henry H.] Goddard used a perversion of Binet's intelligence scale to rank those he considered feebleminded into varying degrees of mental incompetence: idiots (pre-verbal), imbeciles (illiterate), and morons (high-functioning). For Goddard, morons, or those with mental ages of eight through twelve, posed the gravest eugenic threat because of the ease with which they could pass for normal and reproduce. Goddard found morons wherever he looked: criminals, alcoholics, prostitutes, and anyone "incapable of adapting themselves to their environment and living up to the conventions of society or acting sensibly." Most immigrants also fit this classification. Goddard tested immigrants arriving at Ellis Island and found that "[t]he intelligence of the average third-class immigrant is low, perhaps of moron grade.' Goddard concluded that "immigration of recent years is of a decidedly different character from the early immigration .... We are getting the poorest of each race."

Now, some of what made someone an "imbecile" should have been somewhat obvious racially unequal from the beginning - even in 1920, to argue that Black, Latino, or Native American children were receiving a truly equal education to white children was rank hypocrisy. But some of the things that might lead to these issues weren't - right at the time that eugenics was kicking off, we were also putting lead in gasoline and paint - leading to a rise in lead-related mental illness, intelligence loss, and violent behavior. And that rise, along with most environmental problems, affected non-whites far more than white people, because when white neighborhoods complain about toxic waste, it gets relocated to non-white neighborhoods. And redlining just happened to zone black people and industrial use in the same places, resulting in things like Louisiana's Cancer Alley, and the fact that one of the most reliable predictors of life expectancy in the US is your ZIP Code.

Thus, of course, was that the same people clamoring for eugenics standards were also behind governmental and industrial actions that ensured that non-white people were going to be more likely to meet those standards, and worse, those actions led to far more loss of population-wide IQ than a eugenics program could ever save. The open question is how much did they understand these effects. Industry leaders, of course, knew more, because they spent lavishly on PR campaigns to lie to the public. But I personally don't know if the right hand of eugenics and the left hand of environmental and criminal injustice necessarily knew that they were creating a hellish dystopian cycle.

A final thought. From the recent study on lead effects:

We estimate population-level effects on IQ loss and find that lead is responsible for the loss of 824,097,690 IQ points as of 2015.

I also wrote a bit more about the application of the term genocide to the Pol Pot Regime in this answer about whether it was 'a unique form of genocide'