ALTDEV DSCNT THROUGH ASSIGNED ALT.

Narrative

ATC REQUESTED DSCNT TO 11000 FT AND TO SLOW TO 250 KTS. CAPT READ BACK 'DSND TO 10000 FT; SLOW 250 KTS.' ATC DID NOT CORRECT MISTAKE MADE BY CAPT. FO QUERIED CAPT THAT HE WASN'T SURE OF ALT TO DSND TO; BUT CAPT REASSURED FO THAT IT WAS 10000 FT. AT APPROX 10600 FT; ATC ADVISED US TO CLB BACK TO 11000 FT. SUPPLEMENTAL INFO FROM ACN 304307: I HEARD 11000 FT; BUT THE CAPT HEARD 10000 FT AND SET 10000 FT IN THE ALT ALERT INDICATOR. I QUESTIONED THE ALT ASSIGNMENT IMMEDIATELY. THE CAPT SAID NO; HE READ BACK 10000 FT; THE CTLR ROGERED IT; SO 10000 FT HAD TO BE RIGHT. THE BRAND NEW SO LATER ADMITTED TO ME ON THE GND HE THOUGHT HE HEARD 11000 FT. I SHOULD HAVE INSISTED THE CAPT VERIFY THE ALT; BUT I KNEW SUCH A REQUEST WOULD GO UNHEEDED. AT 10450 FT THE CTLR CORRECTED OUR DEV AND I CLBED BACK TO 11000 FT. THE CAPT MUMBLED; 'GUESS YOU WERE RIGHT.' I SHOULD HAVE PICKED UP THE MIKE; AND VERIFIED THE ALT MYSELF. THE MAJOR FACTORS AFFECTING HUMAN PERFORMANCE WERE FATIGUE AND POOR COCKPIT RESOURCE MGMNT. THE NEW SO WAS INTIMIDATED.