there's not much evidence to support that smoking cannabis/hash has any effect on your psyche, but its not very clear cut due to other factors - such as the test subjects use of other things (cigarettes, alcohol, other drugs etc) and their background.
the Guardian did a study and reported that there may be some links to lowered IQ and cannabis addiction, as can persistent, heavy use throughout one's lifetimes cause some negative effects like an impacted IQ or mental health problems.༼ ༎ຶ ෴ ༎ຶ༽
personally, unless one's use of cannabis results in severe mental health problems like psychosis, i don't believe there's any correlation between weed and decreased IQ scores. ¯\_(ツ)_/¯