The immediate incentive completely depends on the implementation of Basic Income, and whether parents of children are given a full, partial, or no Basic Income for each child or the first X children, and in a more macroscopic perspective, flies in the face of consistent evidence that educating and bringing families our of poverty is the best way to get them to have fewer children. First of all, most Basic Incomes plans require citizenship as the only condition, but cash transfer programs in developing countries show that immigration to an community receiving cash transfers increases even if immigrants don't receive the transfer themselves, because the program increases the economic activity in the area. I have to say, I think the misaligned incentives would be less warped than the opposite ones that happen today, in which the chronically-ill may have not enough financial support, and the incentive is to cut their life short instead of trying to take care of them. This is in part because an asymmetric difference exists between taking care of one's own day-to-day needs themselves (much harder to do) and willfully taking one's own life (much easier to do) as a chronically sick person. The other way to look at this is that it would act as a subsidy for automation, as labor costs are purposefully pushed up. The immediate effect might seem to be a loss of productivity, but the long term effect might be the acceleration of the development of automation technology, and longer term gains in productivity.This will be a new incentive to have children. I think having children is great, but perhaps we should think twice before giving people who don't want kids a financial reason to.
There will be greater pressure to slow immigration. I believe that immigration is one of the greatest moral and economic wins for the country, and it should be expanded rather than reduced.
There will be greater incentive to keep suffering, terminally-ill people alive longer, making horrific tragedy more likely.
There is no way to avoid the fact that you are reducing the incentive for people to work. Most people work for money. Some people who now work for their basic income will simply stop. Others will work less, or put less effort into finding work, just like many people getting unemployment benefits now. Less work means less production and less wealth to go around.
Jobs and human employment are an increasingly archaic concept in an age where robotics, automation, technology and science are replacing the human component in the workplace at an ever increasing rate. Time to move forward or be left behind. Try thinking of a UBI as freedom to choose who to spend the limited time life accords us.
Agreed on children. With immigration, I think the established residents will protest (with reason) that newcomers are coming to snatch benefits that they (the residents) paid for. Hurdles like citizenship could reduce the snatching, but many countries provide paths to citizenship for immigrants, and the U.S. offers birthright citizenship. I would prefer not to see immigration opponents have any more ammo. People are less likely to be concerned if the cash transfers are coming from a source outside the country; in that case the boosted economic activity is a win for everyone. On the terminally-ill, of course you are right. Given medical costs, it's hard to imagine it would be profitable to keep someone on life support just to collect a basic income. Your point on automation is interesting. Perhaps this points to a future in which automation (which sounds less farfetched than "robots") provides so many of our needs so cheaply that we will spend most of our time in leisure, pursuing hobbies and consuming entertainment. We have certainly been moving in that direction since the days of the washboard.