As President Obma urges China to be a "source of strength for the community of nations" -- i.e. help with the heavy lifting on international challenges such as global warming and nuclear proliferation -- Chinese leaders prefer to downplay expectations. They're not witholding their support and assistance, but they are parcelling out their contributions quite cautiously, rather than putting themselves at the forefront of global problem solving. Think of it as a tendency to do positive things for negative reasons. Unfortunately, it may not be enough to deal effectively with 21st century international challenges.

I spent several days in Beijing last week taking part in discussions co-organized by the Stanley Foundation with the Centre for International Governance Innovation and the China Institutes on Contemporary International Relations. This Chinese ambivalence about providing leadership was expressed in a variety of ways, including the description of China as "a global actor, not a global power." Indeed, the true aim of key Chinese strategic concepts such as peaceful rise or harmonious world seems to be a frictionless foreign policy to conserve every ounce of effort for the challenges of domestic stability and economic growth.

And that's what I mean by negative reasons. Even when China plays an undeniably constructive role, the main impulse isn't to solve the problem but to stay out of the hot seat of looking like the obstacle. We've all had those fights with a spouse, boyfriend or girlfriend where the other person wants us to do something not to placate them but because we ourselves really want to. They can drive you crazy, those fights. I'm sorry to say it, but here we have a foreign policy equivalent where this actually matters. If China merely tries to keep the United States happy enough, its policy will fall short of the contribution the world really needs.

Take North Korea, for example, where Beijing's other (understandable) consideration is the stability of a brittle and potentially volatile bordering neighbor. It's one thing to calibrate a policy to avoid alienating either Washington or Pyongyang, the apparent current approach, and another thing to push for a denuclearized North Korea through any means of pressure that wouldn't be destabilizing. Chinese leaders need to decide whether their priority is nuclear nonproliferation or friendly neighborly relations because their North Korean counterparts will read the signals, and any tentativeness will in effect serve as a green light for North Korea as a nuclear weapons state. (Similar hard choices between priorities, by the way, go for South Korea and Japan as well.) It's absolutely appropriate to choose means of pressure that will avoid instability in North Korea, but it's counterproductive to inflate such risks and overestimate the impact of some tactics.

I sympathize with Chinese worries that they're not ready for the full weight of attention and expectations to play the role of a global leader. It's just that the world and a set of very high-stakes problems can't really afford to wait. The scene and lighting are set, and the camera is starting to roll.