[1] The effect implies that people tend to select options for which the probability of a favorable outcome is known, over an option for which the probability of a favorable outcome is unknown.
In spite of the equal probability of a favorable outcome, people have a greater tendency to select a ball under option X, where the probability of selecting a winning ball is perceived to be more certain.
The uncertainty as to the number of black balls means that option Y tends to be viewed less favorably.
One possible explanation of the effect is that people have a rule of thumb (heuristic) to avoid options where information is missing.
The effect is often the result of calling some particular missing piece of information to the person's attention.