Fossil records from across the world show that average human brain-sizes have been decreasing for approx. 30,000 years - and especially over the last 10,000. There are various theories regarding the trend (including 'domestication' and inherited genetic abnormalities) but no general agreement. (Note that the reduction in size doesn't necessarily mean a reduction in mental capacity - though since it can't be checked, it can't be ruled out either.)
“[…] the terminal Pleistocene and Holocene (ca. 30,000 years ago to present) witnessed a substantial decline in endocranial volume. This decrease occurred within modern Homo sapiens, and has been observed in many parts of the world. The scope of this decrease is remarkable: for example, within the past 10,000 years the average endocranial volume in European females reduced from a mean of 1502 ml to a recent value of 1241 ml. This decrease of approximately 240 ml in 10,000 years is nearly 36 times the rate of increase during the previous 800,000 years.”
Source: Selection for smaller brains in Holocene human evolution , arXiv, 2011.