A cargo company Captain reported the First Officer could not keep the aircraft on centerline during takeoff so the Captain took control and rejected takeoff.

Narrative

First Officer's Take off; Seconds into the takeoff roll on runway 31 my FO states; (paraphrasing) 'I can't control the airplane.' Initially I was puzzled because everything looked and felt normal to me. He then stated (paraphrasing); 'I have full left rudder and can't hold the center line.' The inflection in his voice indicated he was dead serious. Suspecting a flight control problem; I responded with 'Reject; I have the aircraft.' Control of the aircraft was transferred back to me. I could feel the asymmetry in the rudder pedals with the left pedal all the way to the floor. We conducted a low-speed RTO at approx. 60 knots. We exited the runway onto taxiway 1 and conducted the RTO (Rejected Takeoff) and after landing checklist. My FO stated he felt the aircraft wanting to aggressively drift to the right on takeoff and he had the left rudder pinned to the floor to maintain control. My hand was on the tiller at the time; but control input was minimal; if any. In no way; shape or form did I feel I was counteracting a hard rudder input with the tiller. We coordinated with ground control to park in a place in which we were not in anyone's way. We contacted operations and asked our dispatcher to patch a 3-way call to Maintenance Control. During the conference call my FO spoke with Maintenance Control so he could accurately describe firsthand what he felt the aircraft was doing. Maintenance Control recommend going back to the gate for a series of inspections. After block in we spoke with the duty officer and made a MEL write up. Cause: Unknown. The MX inspections did reveal anything outside normal parameters. Suggestions: Considering the unexpected nature of the event; the critical phase of the flight and compressed time frame (seconds) to observe; orientate; decide and act I feel we did a good job. My FO; based on 12000 hours of flight time; felt something didn't seem right and feel right. He did the right thing by speaking up in an assertive manner. This was the first time in my career I've had a First Officer state 'I can't control the aircraft;' on his take off roll. I felt the correct response on my part was not 'Continue' but instead 'Reject; I have the aircraft.' We were below 80 knots (low speed regime). Delaying the RTO (Rejected Takeoff) to diagnose the problem while accelerating would have created more jeopardy. I realize my RTO (Rejected Takeoff) and our block-turn-back created significant delays. Additionally; it tied up a lot of men and resources to do their due diligence and inspections. I'm aware. However; our actions reflected what we are taught in training. At the end of the day a lot of people were inconvenienced but there was no bent metal and no injuries.