Well, that requires one to know wth are the Fibonacci numbers or be able to figure out they follow whatever sequence they do, which is much more likely if someone has been explained the Fibonacci numbers, or at least done math beyond “one, two, many”. That’s environmental without even needing to be “training to beat the test”.
I know anecdote isn’t the plural of data, but…
My mom’s siblings all have, at minimum, master’s degrees. Two of us in my sibling group have doctorates and virtually all of my cousins have advanced degrees (and three are professional opera singers).
On my dad’s side, virtually no one went to college. I’m not arguing no one is smart on that side of the family, but in comparison they live rather limited lives in terms of new experiences/willingness to try new things.
Both sides of the family come from dirt poor/working class roots (none of my grandparents went past eighth grade).
So…maybe some genetic inheritability, family cultures, and so on. A central difference in environment is we are all voracious readers on matrilineal side; dad’s family takes TV Guide and that’s about it (plus that glorious literary work, The Book of Mormon).
Thanks for the laugh.
Some people just don’t get sarcasm 
Do you mean Gibbon’s Decline and Fall of the Roman Empire? If so I’d need some convincing, that work consists of 79 huge and complex chapters, the whole consisting of some 1.5 million words. Possible? Perhaps. Unlikely? In the extreme.
It turns out that the more variable a brain is, and the more its different parts frequently connect with each other, the higher a person’s IQ and creativity are.
But we haven’t even agreed on what type of ‘smart’ we are talking about. No, I’m not talking about physical coordination vs. social interactions vs. musical creativity and so forth.
What about speed of thinking vs. breadth of knowledge vs. depth/power of thinking?
I’d guess experience and training might affect the speed part the most. A chess master takes much less time to decide how best to proceed in a game situation than a novice does: he can eliminate whole swathes of possible lines as unpromising right away, because he’s worked through them in the past.
Breadth of knowledge is probably the most easily affected and where nurture and environment come in. A player who has had access to books about chess play and the time/inclination to study them will know how to use strategies someone without those resources has no idea exist.
But depth/power of thinkig, the sheer ability to work out something new, to figure things out beyond what has been shown/taught you, to come up with a new solution to a problem… I think that must be purely genetic. Being able to hold the positions of the board in your mind, ‘look’ at what they will be if you do X, and then what if the other guy does Y, and then you do Z… That seems like it might depend on the pure ‘hardware’ of the brain.
Actually, that seems sort of like a computer analogy would fit. How good is the programming vs. how much data is stored vs. how fast can you flip them 0s and 1s.
Almost like the question - “Who’s stronger? A man who can lift 400lb or a 90-lb woman who can run a marathon in competitive time?” Presumably IQ is just a rough attempt to summarize the mental version of a decathlon tryout…
I read a description one of a fellow in Stalinist Russia who had a “photographic” memory. When someone challenged him on the minutes of a meeting he had typed up, for example, he could recite the conversation from the meeting verbatim. however, he was described as boring and unimaginative.
Similarly, Aspergers or other autistics may have good or even better abilities in some brain function areas but lack in others. A single adjective or a single number can’t truly summarize a complex collection of capabilities. Like Jennshark, I am the least educated of my siblings or parents (only a BSc), yet I haven’t see the same imaginative spark in them.
Eric Raymond had an interesting essay on this topic. His claim is that autistics can exhibit brilliance in narrow areas because they are using brainpower that most people allocate in other areas, like social awareness:
Back when I was working at Drexel U. in Philly, I had a carburetor on my desk. A Penn professor walked in to see my boss. “Hi, Mitch! What’s that?”
“Hi, Alan. Mercedes carburetor. I just overhauled it!”
“Oh, nifty! Never could do that, myself.”
With that, Dr. Alan MacDiarmid, Nobel Laureate in Chemistry, gave me a cheerful wave and went in to see my boss. 
-MMM-
My dad, besides being brilliant in his field, also used to maintain his own British motorcycle in university. He would rebuild throwaway parts like a mechanical voltage regulator. He an incredibly practical. I once watched him make a new blade for his coffee grinder from an old piece of metal bedspring slat, including sharpening and tempering. He was in demand as both a theorist and designed of experiments and went all over the world. He once made a timer device with pulley out of a cheap meccano set gears and clock drive when the equivalent from a scientific supply house was close to a thousand dollars. (He was a serious cheapskate too) Yet others in his department were not so good - one prof almost cut his thumb completely off using the bandsaw…
Partly talent, partly deep interest, partly practice. I rebuilt a carburetor once - for a VW Beetle - by following a book. It worked. (Did you know if you reverse the spark plug cables on a Beetle, it will actually run OK until you approach 25mph? )
So to avoid the defining “smart” aspect, let’s just ask “why do people have different cognitive abilities?”
And there the answers are the same as the broader phrased version yet “why do people have different abilities?”
Answer one. It is multifactorial. A combination of genetic predispositions and environmental impacts. Some are born with more innate musical abilities, some with more social smarts, some more creativity, some more quick-witted, some with verbal acumen, some better visuospatially, some with reading disabilities, some a tendency to focus intensely and some with a tendency to think about ten things at once and make the connections. Some are born into families and cultures that allow their specific gifts to fully blossom but that would not fully nurture another set of gifts and some, not. As parents we always want to follow the mantra of “don’t block the blessing” but let’s face it: we do not always succeed perfectly.
Which segues into answer two. We are social creatures. We function as members of teams, be that unit just our family, our tribe, or broader society. There are niches for a variety of generalists and specialists to varying degrees and having that variety of differing sorts of cognitive abilities and processing styles as part of our group makes our group stronger.
That would be Imperial Oil Limited (no such company as “Esso Canada”) which is majority owned by Exxon Mobil. Imperial owns or franchises the Esso stations and is a major player in Canadian oil exploration and development. Many years ago I heard that they were exceptionally well managed and seemed to distinguish themselves from their parent company with exemplary corporate citizenship. I have no idea how much that has changed.