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Cross-sectional theory is when the studies are being observed over different ages, they are able to see the outcome immediately.
Longitudinal is when the same test is being conducted with the same individuals over time and the conclusions are drawn out.

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Theories help aid in testing out not only the "why, but how" of any observed actions. The amount of of times each theory is tested and concluded with the same outcome, it becomes more prevalent in use.

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SeNsiTive/cRiTiCAl PeRIods:
When it comes to ethical research in humans, we have to be mindful of two things; sensitive and critical periods. There are some things that we only seem to understand at a certain time in life (sensitive) and others when it is near impossible to grasp afterwards (critical).

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By first choosing an area in which they'd like to focus, scientists study people by observing an individual(s) in person, behind a wall, and/or using recording devices. An advantage of observing in person gives an up-close look into the subject being observed e.g.( how they interact individually and with others ). But this also is a disadvantage because the subject's behavior can be altered by its environment  and surroundings.
1. experimental- variables cannot always be controlled. 2. descriptive- once an individual becomes adapted to the observer, they may allow the real "them" to show in their natural habitat.
3. correlation: there is usually a relationship between what's being observed.