That study is from the government.
You do know the governement claims marijuana is bad for your health, but has a patent for medical use right? That's hyprocrisy, so I'm going to take all their so call scientific studies with a grain of salt. It's a fact using a lack of motivation is flawed premise. You can literally correlate anything and say it leads to a lack of motivation.
Why? The term "lack of motivation" is open ended (think the term terrorism), so you can configure the meaning in whatever way you want to fit your premise.
Another blemish in your intelligence Mr. Doctorate.