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DanArmak comments on How inevitable was modern human civilization - data - Less Wrong
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We don't know that a larger brain is required for greater intelligence (in e.g. birds).
I agree. But it's "easier" to evolve a slightly larger brain with the same architecture, than to discover a new, more efficient architecture.
In general we should expect a larger brain (ours consumes 1/4 of our total energy) to pay for itself in more actual intelligence; if it doesn't (the architecture can't easily scale) then the smallest (still working) brains will win.
For each species of bird, either
1) general scalable intelligence isn't supported by their brain's architecture (so you can't just grow more processing power)
2) birds lack the physical attributes to profit from any more general intelligence than they already have
or
3) it (general intelligence) just hasn't evolved yet
I don't think we should generalize from the single data point of recent human evolution. Are we sure larger brains tend to give greater general intelligence in other lineages? Has this been checked? Is the intra-species brain size variation large enough to check this in species where we already run intelligence tests?
The fact that human brains recently became unusually large can be explained by other theories. For example, it seems more likely to me that first there was a (relatively recent) mutation that significantly changed brain architecture, and only after that point did the new brains profit from growing larger. (E.g., chimps would have come before this mutation, and indeed have not experienced runaway brain growth.) On this theory, brain size only benefits the lineage with a very specific neural architecture and isn't a general rule.
There have also been other theories, some of them invoking sexual selection.
Most of the brain does tasks not directly related to general intelligence, e.g., lower-level input processing, managing the digestive system, etc.
Increases in brain size might also improve these functions and be an advantage in their own right. This would further muddy the issue if we charted intelligence vs. brain size, because we don't know enough yet to say which parts of the brain (particularly of a non-human brain) have which general-intelligence functions. Bird brains, for instance, aren't much like mammal brains, they don't even have a neocortex, so we can't just compare growth of brain areas directly. (Not to mention cephalopod brains...)
Finally, brain size directly correlates with head size, and most of the time I expect head size would be more evolutionarily significant. It controls things like eating (size of mouth and throat), defense (size of teeth and jaw), acuteness of the senses (eye size)...
Not to nitpick, but the definition of fully general intelligence is that pretty much anyone can benefit from it.
Is there any evidence that fully general intelligence exists in this world?
Depends on the definition used. You could argue that Bayes' Law is fully general intelligence.
Bayes Law is a fact/theorem which is probably useful for anyone who can understand it. But is that what you mean by intelligence? I thought it was about the abilities of an individual.
OK, then, any individual understanding Bayes' Law could be said to have "fully general" intelligence.
This is silly.
Silly? But is it true or false?
Larger brain of the same architecture.
That's what I'm talking about. What reason do we have for thinking that larger brains of the same architecture exhibit more general intelligence (in non human lineages)?
Also, what exactly does it mean for two brains to have the same "architecture" if they differ by a genetic mutation? It's not as if there's a separate gene coding for "brain size" that could mutate on its own.
If it doesn't help, and uses more energy, then it won't get kept unless it's an inevitable side effect of something helpful. That was my only basis for "larger (of same type) => more intelligence".
I don't really know anything about this topic. My claim is essentially a tautology that may not have much practical application.
Some trivia (not directly related to my original claim) I found at http://en.wikipedia.org/wiki/Brain_size:
That's true. But something helpful done by the brain isn't necessarily involved with intelligence.