1. Sampling bias.
May not reflect all of population. It is costly to gather people from
all around the world, just to do an experiment (and hard to convince
participants too, for experiments that need physical presence)
2. Due to the law, research on people under 18 typically needs parental
or school district consent. In such a fast-paced world based on
convenience, and schools' goals of not gaining any bad PR, it's pretty
much going to be a near-universal rejection on consent to research.
Thus, there is little research done on teenagers in particular, despite
their mental health struggles! This is why the education system remains alive!!
3. Showing a correlation can be misleading. This is because this does
not mean a direct causation!
For instance, some say people who sleep more die earlier. In reality, it's just that in
the last few weeks, people get very tired/sick and, of course, want to
sleep more, resulting
in a higher average sleep compared to people who overall have a higher
lifespan and thus spend less time (relative to their total lifespan)
asleep.
As another example, some say that social media causes
depression. In reality, it could be that schools are the main
perpetrators, as depression rates are lower in the summer, and without
school, it's likely that teenagers will spend even more time on social
media! Rather, social media can be a way to escape stress and
depression! Even though it doesn't necessarily improve anything, the
free act of watching videos likely does not cause that much discomfort,
so such a relation is questionable.
4. People sometimes research obvious things. Most research is monetized.
Most research thus supports their initial claim; otherwise, it implies
they failed at it.