Analysis of cluster and field star uvby data demonstrates the existence of a previously undetected discrepancy in a widely used photometric metallicity calibration for G dwarfs .
The discrepancy is systematic and strongly color-dependent , reducing the estimated [ Fe/H ] for stars above [ Fe/H ] \sim - 0.2 by between +0.1 and +0.4 dex , and creating a deficit of metal-rich stars among dwarfs of mid-G and later spectral type .
The source of the problem , triggered for stars with b - y greater than about 0.47 , appears to be an enhanced metallicity dependence for the c _ { 1 } index that increases as temperature declines .
The link between c _ { 1 } , normally a surface gravity indicator , and metallicity produces two secondary effects .
The deficit in the photometric abundance for a cool dwarf is partially compensated by some degree of evolution off the main sequence and cool dwarfs with metallicities significantly above the Hyades are found to have c _ { 1 } indices that classify them as giants .
The potential impact of the problem on stellar population studies is discussed .