I think it should be the student with low of inefficient level of dopamine because band there will be reduction in some cognitive function and dopamine also excite the nerve fiber with low level this will be reduce leading to craving for stimulants

I'm not sure why you want to focus on hypothalamic function to determine the expected outcome, but the more opinion the better. Hypothalamus is more often discussed when discussing about autonomic system. Usually, when we are talking about impulsivity, the focus would be more about prefrontal cortex, striatum, mesostria pathway, or corticolimbic pathway.

Prefrontal cortex governs salient function, like decision making. If the processing in prefrontal cortex is suboptimal and the individual prefers instant reward (instead of reward after proper thinking), probably via stimulation from mesolimbic or mesostria pathway, it would result in impulsivity. As has been mentioned, impulsivity can result from imbalance of dopamine signalling. Individuals with substance abuse are often reported to have low dopamine D2 receptor, recep@tors which is assumed to be controlling "punishment" response in mice. Mice with forced activation of D2 receptor would freeze and stop moving. However, others have speculated that D2 receptor is a receptor that, when activated, causes reward dependency and inability to move without strong reward (this opinion contrasts the lack of D2 in substance abuser). But at least most have agreed that dopaminergic dysfunction is involved, in one way or another. However, it's doubtful which of dopamine receptor is "good" receptor, as overactive D2 stimulation is thought to be the possible cause of schizophrenia.

Indeed, hypothalamus has a role in governing reward, although I'm not sure to what extent. As for dopaminergic neuron, the function is not based on "the size of the neuron." Neurons create new connections over time, the ones called synapse. They will then trim unnecessary connection, by synaptic pruning. In this sense, optimal neuron network can be imagined as large network (a single neuron can connect to thousands others) with efficient pruning instead of a number of individual neurons with large and effective function. However, again, this needs to be interpreted cautiously as excessive pruning can also cause schizophrenia. Dopamine does not appear as a pool with concentrated substance, but instead, is kept inside the neuron. It will only be released very briefly when a neuron needs to communicate with it's postsynaptic partners. So the answer to whether or not brain concentration in two individuals can be considered similar or different is ambiguous at best.

I hope it helps. If I make any mistake, feel free to correct me.

Thanks for the reply, Afkar Aulia.

I'm focusing on the hypothalamus since its plasticity and the possibility of it changing according to a generational cycle. (See my research project for more info on this.)

My hypothesis is that population cycles are driven by changes to different parts of the hypothalamus, which is why the size and thus secretion efficiency matters, as it determines the strength of the physiological and behavioral effects (on a general level). So think of the secretion efficiency on a generational level in a population, which is the reason why I asked the question in the first place.