People with scientific backgrounds hypothesize all the time, and they have machines that can make brain scan images, but we aren’t very close to being able to measure addiction on a numerical scale.
The best we have is listening to people, maybe analogous to how we listen to people when they believe they are a different gender, etc. since there is no quantitative measure for ‘feelings’ (see Olympic male/female athlete controversies, et al.)
We may get there some day...
Hard sciences may be, but there are quite a soft science domains such as psychology. We do not yet know exact nature of addiction, but psychological, environmental impacts have not been ruled out, by most scientists.
DeltaFOSB is a gene transcription factor and is the precursor for the development of all forms of what is clinically called "addiction". However what the other commentators suggested [food is not addictive] is not actually true, it has a measurable affect here. In fact, Delta-FOSB can be and often is used as a biomarker of addiction in patients who are already deceased (and can have their brains broken open).
Now, as to the question of addictiveness, nobody mentioned anything about pornography there, but I did hear an interesting anecdote about the raw power that certain addictions have that clearly isn't present in pornography; Delta FOS-B accumulates in the ventral tagamental area and nucleus accumbens, two central regions of the brain, and many drugs have the action of creating floods of FOS-B precursors there, for example, this effect is so strong in heroin addicts, that they nearly always throw up upon recent re-exposure (sometimes newer Methamphetamine addicts as well) where the mechanism here is the flood of FOSB, which an outer region (she had mentioned but I have already forgotten which) interprets as "poisoning". So while it may be true that pornography is "addicting" - when is the last time you saw someone throw up when they looked at pornography (modulo ordinary "disgust").
What was doubly interesting was that she said the change in dopamine regulation (meaning the introduction of new dopamine-like substances + the change in the rate that your body "reuptakes" dopamine) in the most powerful addictions -- benzodiazapines and opiates, aren't actually that high, and compared with drugs like methamphetamine and cocaine aren't much different from marijuana.
It's this secondary affect that creates these changes in reasoning and reward structure with these drugs (she also mentioned that both have "withdrawal", which while technically unrelated to addiction is extremely painful and sometimes life-threatening).
However, to be fair, it was repeatedly stressed that the "secondary" affect was more than just Delta-FOSB. There were changes in signaling, new kinds of peptides that unaddicted people can't produce (???), something about phosphoration of camp ("new notes on camp") and bunch of change to "Dendritic Spiny Structures".
Among the living, questions like "drug-use reinstatement likelihood", "place conditioning preference" (the more addictive the drug, the more you want to use it in a safe, known place), increase in use over time, etc. are how this stuff is generally measured with reasonable economics.
This is not precise, but it seems like an appropriate metric. It's basically the same as defining mental illness in general vs. harmless eccentricity.
Many people will also say that a delicious food, an entertaining game, or an enjoyable TV show is "addictive", but these do not actually constitute addictions either.