From Serendip
Brain Size and Evolution - Where we are ... so as to see where we might go next | 
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Some conclusions:
- Brain size is not, in and of itself, a particularly interesting measure, but ...
- Brain size is lawfully related to body size. "Why?" is an interesting question.
- Across species, brain size relative to body size seems to be related to a poorly defined but intuitively meaningful concept of "behavioral complexity". The sigificance of the intuitive concept and of the correlation are both interesting questions.
- Brain size (and relative brain size) has increased during evolution in a variety of different lineages.
- In many cases, these increases are increases in the range of the size distribution rather than involving replacement of smaller brains by larger ones.
- Larger (both absolute and relative) brains are not , in general, "better" than "smaller" brains in any meaningful biological sense.
- Evolution likes to "play", and this "playfulness" probably accounts to some extent for the "directionality" apparent in the evolutionary record. Neither bigger (nor more complex) necessarily means "better".
- Brain size has dramatically increased in the human lineage, apparently with a general trend of larger-brained organisms replacing smaller-brained ones. Is this more "playfulness" or is it "selected for"?
- Brain size increase in the human lineage seems to represent a continuation and acceleration of a vertebrate trend toward increased relative size of a particular subsystem of the brain: the neocortex and related regions.
- The neocortex may be particularly significant in "playing", i.e.in both generating and dealing with ambiguity/unpredictability.
- Increases in playfulness may create a selection pressure for neural systems better able to create/deal with playfulness, creating an evolutionary positive feedback loop.
- Positive feedback loops provide another way of accounting for directionality in evolution. In this case, the selection pressure is generated by evolutionary change itself, and "adaptedness" is entirely context dependent.
- "Behavioral complexity" may relate to the extent to which the brain functions as an "amplifier of evolution".
Some links for further exploration:
- Intelligence, brain size, and evolution, from Scientific American's "Ask the Experts"
- The Cerebral Cortex, or My Brain is Bigger than Your Brain, from Neuroscience for Kids
- Allometry, a critical exposition and discussion using vertebrate brain size as the subject
- Controversy expands over evolution and brain size, a news report on fossil findings, from Nando Times
- Does brain size matter?, two articles on the brain size/intelligence issue in extant humans, from the Canadian Journal of Experimental Psychology
- Brain Size, a Human Prehistory module from a World Civilizations course at Washington State University
- The Evolution of Primate Intelligence, from the Harvard Undergraduate Journal of Neuroscience
- Skoyles on brain size and expertise, a Psycoloquy article on, among other things, human microcephaly
- Paleoneurology: the study of brain endocasts of extinct vertebrates, by Harry Jerison
- Comparative mammalian brain collection, University of Wisconsin
- William Calvin's home page, lots of materials, links, and thoughts about brains and evolution (among other things)
Some interesting books:
- Christopher Wills, The Runaway Brain, 1993
- Merlin Donald, Origins of the Modern Mind, 1991
- Mitchel Resnick, Turtles, Termites, and Traffic Jams, 1994
- Stuart Kaufman, The Origins of Order, 1993
- John Tyler Bonner, Evolution of Complexity, 1988
Want to play/evolve some more?