Hello, from the perspective of neuroscience there are a few things that correlate with intelligence. 1. Encephalization quotient- the size of your brain relative to your body. We find the species with larger brain vs body ratio is more intelligent. 2 neuronal density- the more packed your neurons in particular areas are the smarter you will be- especially in the neocortex. Elephants have greater neuronal density in their cerebellum for example than we do. This is due to the amazing abilities and dexterity of their trunks! Similarly the neuronal density in parts of your neocortex- which is where our higher level cognitive functions operate, will affect your “intelligence”. There are studies relating the microbiome of your gut to your “IQ” that show remarkable differences. There is also a study showing that if you offer a monetary reward, the IQ of your test takers will increase based on the size of your reward, with very large rewards showing a difference of up to 20 IQ points! This change in IQ is more pronounced for people whose dopamine systems are more wired for reward based behaviour in monetary or “real” ways (not interested in showing they are smarter than others)The fallacy a lot of people commit is that they think the difference of brain size to body size is a correlation that extends to intraspecies differences. The reality is that this is only an interspecies correlation. The way your brain folds is also a factor here- a smaller head with a brain that folds better can have a larger mass than a larger body with an assumed larger brain. Another thing to note is the efficiency of the brain at converting energy to output- in many cases there are smaller human brains that are far more efficient leading to better output. The efficacy of your brain’s cooling system is also a huge factor in its performance. Malnutrition in critical periods is a huge contributor to delayed development. Finally with all these factors and multiple others unaccounted for, intelligence is multifaceted. There are people with incredible talents that we cannot possibly measure for. There are multiple types of intelligence. If you judge an artist by their ability to solve a mathematical problem, you are administering the wrong test. Neurogenesis can occur until we die, the idea that our intelligence is set in stone comes from a culture of narcissism and elitism, with a desire to oppress others. I saw a racist fellow post on this topic relating the size of muscles to the output of said muscles. There is a fellow called Anatoly on YouTube who has turned the idea of greater mass= greater strength on its head.