There are three components to the variance: genetics, shared environment, and non-shared environment (noise basically). Shared environment is almost immeasurabl

There are three components to the variance: genetics, shared environment, and non-shared environment (noise basically). Shared environment is almost immeasurable for most of these traits (under 10%) while most is the variance is due to genetics or non-shared environment. The way non-shared environment is determined is by subtracting genetics and shared environment from the total varience. The worse the test the bigger the component that ends up in the non-shared environment factor. The error bars depend on the trait being measured. Some are relatively small (like g), while others like "agreeableness" (just choosing a random trait) will be quite large as it is hard to measure this accurately and consistently.