user3149: For example, take a Class A (a player with a USCF rating of between 1800 and 1999 inclusive) today and Class A from the 90's. Would the contemporary Class A be stronger because of the updates of opening theory or contemporary changes in chess play?

user5653: They are more equipped with chess technology and have a bigger circle of experts around to hire/train with. 
This means that contemporary players have better opportunities than players before their time. 
Hope this helps.

user9684: https://en.wikipedia.org/wiki/Flynn_effect is a theory that we, on average, are getting smarter.
"Ulric Neisser estimated that using the IQ values of 1997, the average IQ of the United States in 1932, according to the first Stanford–Binet Intelligence Scales standardization sample, was 80."
"Trahan et al. (2014) found that the effect was about 2.93 points per decade, based on both Stanford–Binet and Wechsler tests; they also found no evidence the effect was diminishing."
If chess performance has any collaboration with intelligence then yes. 

user4399: What do you mean by 'strength'?
Do you mean talent (IQ, natural ability, genius, w/e; Re IQ, see 1, 2 ft Josh Waitzkin, 3) adjusted for time, resources, opening theory? That's probably a statistics, biology or cognitive science question. If the answer is yes, I don't see why that wouldn't hold true for math, basketball or singing.
If you mean talent unadjusted for such, then obviously today's top players are 'stronger'.
If you're talking about chess enthusiasts, probably not, but who's to say for sure? If you're talking about GM-level play eg Anand vs Morphy, definitely. If you're talking about NMs/IMs of today versus NMs/IMs (or their equivalents) of before, probably.
An analogy would be saying that Andrew Wiles is a 'stronger' mathematician than Taniyama, Leibniz or Euler. Of course e is, but that's mainly because of the works of the ones who came before him.
As Isaac Newton said:

If I have seen further, it is by standing on the shoulders of giants.

Anyway, going back to chess
Additionally, Bobby Fischer said (emphasis mine):

In chess so much depends on opening theory, so the champions before the last century did not know as much as I do and other players do about opening theory. So if you just brought them back from the dead they wouldn’t do well. They’d get bad openings. You cannot compare the playing strength, you can only talk about natural ability. Memorization is enormously powerful. Some kid of fourteen today, or even younger, could get an opening advantage against Capablanca, and especially against the players of the previous century, like Morphy and Steinitz. Maybe they would still be able to outplay the young kid of today. Or maybe not, because nowadays when you get the opening advantage not only do you get the opening advantage, you know how to play, they have so many examples of what to do from this position. It is really deadly, and that is why I don’t like chess any more.

Magnus Carlsen seems to think he could defeat Fischer or Tal, and why wouldn't he? He knows their games, has the Internet and is granted opening theory that Fischer, Tal and others had to discover themselves (eg 1 2).
As chess.com user fabelhaft put it (emphasis mine):

the main thing is that there is very little difference in talent between the best players of any time. The big difference is in the opportunisties the players had. Without professional coaching since childhood, modern opening theory, engine assistance, game databases, tournament circuits, Internet, etc, it was just impossible to play on the same level as those with all these advantages that were similarly talented.


user8751: I don't think that there is a significant difference at that level. The main difference with chess 20 years ago is the use of computers and they are not extensively used by club players. 
Also the advances in opening theory have been profited mostly by the top players (a club player normally doesn't know a lot of theory and doesn't memorize a lot of moves in depth of each variation). 
Furthermore you can consider rating inflation and now the new K factors which make easy for a young player to improve his rating very rapidly in a good tournament.
In conclusion, I don't think that 1800-2000 players are stronger now than 20 years ago but I do think that top players are stronger now because of the facts mentioned above.

user924: No. The vast majority of club level players still play like the vast majority of club level players always did. Changes in knowledge about chess mostly apply to high levels of play; the kind of mistakes class A players make were already known to be mistakes for ages, and opening theory doesn't help much if your opponent deviates from theory at move 5 anyway.
As evidence of this, see the paper "Intrinsic Chess Ratings" by Regan and Haworth, PDF. They analyzed the games of players of various rating ranges from various time periods with an engine, and found no significant difference in the mistake rate between games from the 70s and from the 2000s.

3149: <p>For example, take a Class A (a player with a USCF rating of between 1800 and 1999 inclusive) today and Class A from the 90's. Would the contemporary Class A be stronger because of the updates of opening theory or contemporary changes in chess play?</p>
 5653: <p>They are more equipped with chess technology and have a bigger circle of experts around to hire/train with. 
This means that contemporary players have better opportunities than players before their time. 
Hope this helps.</p>
 9684: <p><a href="https://en.wikipedia.org/wiki/Flynn_effect" rel="nofollow noreferrer">https://en.wikipedia.org/wiki/Flynn_effect</a> is a theory that we, on average, are getting smarter.</p>

<p>"Ulric Neisser estimated that using the IQ values of 1997, the average IQ of the United States in 1932, according to the first Stanford–Binet Intelligence Scales standardization sample, was 80."
"Trahan et al. (2014) found that the effect was about 2.93 points per decade, based on both Stanford–Binet and Wechsler tests; they also found no evidence the effect was diminishing."</p>

<p>If chess performance has any collaboration with intelligence then yes. </p>
 4399: <p>What do you mean by 'strength'?</p>
<p>Do you mean talent (IQ, natural ability, genius, w/e; Re IQ, see <a href="https://medium.com/incerto/iq-is-largely-a-pseudoscientific-swindle-f131c101ba39" rel="nofollow noreferrer">1</a>, <a href="https://www.creativitypost.com/article/joshua_waitzkin" rel="nofollow noreferrer">2 ft Josh Waitzkin</a>, <a href="https://www.khanacademy.org/college-careers-more/talks-and-interviews/talks-and-interviews-unit/conversations-with-sal/a/the-learning-myth-why-ill-never-tell-my-son-hes-smart" rel="nofollow noreferrer">3</a>) adjusted for time, resources, opening theory? That's probably a statistics, biology or cognitive science question. If the answer is yes, I don't see why that wouldn't hold true for math, basketball or singing.</p>
<p>If you mean talent unadjusted for such, then obviously today's top players are 'stronger'.</p>
<p>If you're talking about chess enthusiasts, probably not, but who's to say for sure? If you're talking about GM-level play eg Anand vs Morphy, <strong>definitely</strong>. If you're talking about NMs/IMs of today versus NMs/IMs (or their equivalents) of before, probably.</p>
<p>An analogy would be saying that Andrew Wiles is a 'stronger' mathematician than <a href="http://mathworld.wolfram.com/Taniyama-ShimuraConjecture.html" rel="nofollow noreferrer">Taniyama</a>, Leibniz or Euler. Of course e is, but that's mainly because of the works of the ones who came before him.</p>
<p>As Isaac Newton said:</p>
<blockquote>
<p><a href="https://en.wikipedia.org/wiki/Standing_on_the_shoulders_of_giants" rel="nofollow noreferrer">If I have seen further, it is by standing on the shoulders of giants.</a></p>
</blockquote>
<p>Anyway, going back to chess</p>
<p>Additionally, Bobby Fischer said (emphasis mine):</p>
<blockquote>
<p>In chess <strong>so much depends on opening theory, so the champions before the last century did not know as much as I do and other players do about opening theory. So if you just brought them back from the dead they wouldn’t do well. They’d get bad openings</strong>. You cannot compare the playing strength, you can only talk about natural ability. Memorization is enormously powerful. <strong>Some kid of fourteen today, or even younger, could get an opening advantage against Capablanca, and especially against the players of the previous century, like Morphy and Steinitz</strong>. Maybe they would still be able to outplay the young kid of today. Or maybe not, because <strong>nowadays when you get the opening advantage not only do you get the opening advantage, you know how to play</strong>, they have so many examples of what to do from this position. It is really deadly, and that is why I don’t like chess any more.</p>
</blockquote>
<p><a href="http://www.chess.com/news/carlsen-i-would-beat-tal-and-fischer-4554" rel="nofollow noreferrer">Magnus Carlsen</a> seems to think he could defeat Fischer or Tal, and why wouldn't he? He knows their games, has the Internet and is granted opening theory that Fischer, Tal and others had to discover themselves (eg <a href="https://en.wikipedia.org/wiki/Veniamin_Sozin#Sicilian.2C_Sozin_Attack" rel="nofollow noreferrer">1</a> <a href="https://en.wikipedia.org/wiki/King%27s_Gambit#Fischer_Defense:_3...d6" rel="nofollow noreferrer">2</a>).</p>
<p>As chess.com user <a href="https://www.chess.com/news/view/carlsen-i-would-beat-tal-and-fischer-4554" rel="nofollow noreferrer">fabelhaft put it</a> (emphasis mine):</p>
<blockquote>
<p>the main thing is that there is very little difference in talent between the best players of any time. The big difference is in the opportunisties the players had. Without <strong>professional coaching since childhood, modern opening theory, engine assistance, game databases, tournament circuits, Internet</strong>, etc, it was just impossible to play on the same level as those with all these advantages that were similarly talented.</p>
</blockquote>
 8751: <p>I don't think that there is a significant difference at that level. The main difference with chess 20 years ago is the use of computers and they are not extensively used by club players. </p>

<p>Also the advances in opening theory have been profited mostly by the top players (a club player normally doesn't know a lot of theory and doesn't memorize a lot of moves in depth of each variation). </p>

<p>Furthermore you can consider rating inflation and now the new K factors which make easy for a young player to improve his rating very rapidly in a good tournament.</p>

<p>In conclusion, I don't think that 1800-2000 players are stronger now than 20 years ago but I do think that top players are stronger now because of the facts mentioned above.</p>
 924: <p>No. The vast majority of club level players still play like the vast majority of club level players always did. Changes in knowledge about chess mostly apply to high levels of play; the kind of mistakes class A players make were already known to be mistakes for ages, and opening theory doesn't help much if your opponent deviates from theory at move 5 anyway.</p>

<p>As evidence of this, see the paper "Intrinsic Chess Ratings" by Regan and Haworth, <a href="http://www.cse.buffalo.edu/~regan/papers/pdf/ReHa11c.pdf">PDF</a>. They analyzed the games of players of various rating ranges from various time periods with an engine, and found no significant difference in the mistake rate between games from the 70s and from the 2000s.</p>
