based on common logic - when ppl chase up the price of something, the yield (based on price that U bought in mar) goes down or in changgih talk "yield compresssion".
ppl "fear" equities, thus move/hold in bonds/bond funds, thus... thus... thus the above

my 2 cents, not gospel truth 

Note - if U bought several years ago and holding, still getting similar DY% as last time right? Coz your cost was those days, not nowadays' prices

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Added on December 28, 2012, 1:25 pm
Tabulating my net worth and investment assets + seeing them grow MONTHLY is part of my entertainment & leisure
I digress though hehe - balance i think, is best unless U want your "naughty kid"-side to come out and bomb your savings/investments. Seen this happening in friends and myself - the "what about me!" (fun seeking kid side) causes splurges and "accidental investments" when i didn't give balanced attention to "him"