Why give a child a sticker? The typical answer touches on motivation, positive praise, the acknowledgement of good behaviour or good work, the encouragement of non-compliant pupils to stick to rules or to stay on task. Giving out stickers is accepted practice, not in all schools, but in lots of schools. So why's this a problem?
Children's motivation to engage in learning should be intrinsic, driven by their own internal sense of enjoyment and fulfilment, by what Ryan and Deci describe as the "psychological needs for competence and autonomy". Teachers fuel children's intrinsic motivation by providing high quality learning experiences (lessons, activities, resources), by offering a relevant and/or stimulating curriculum, and by fostering individual pupils' sense or understanding of their own progress. When motivation is intrinsic, rewards are unnecessary. Let me expand.
a) If the lesson/activity/resource is interesting, purposeful and challenges everyone at an appropriate level, then pupils will be motivated. If you recognise and address the barriers that may impact upon individual pupils' engagement, then all pupils will be motivated. You don't need stickers - the lesson is a reward in itself. If, on the other hand, the lesson is dull, unchallenging, and/or involves the completion of mundane, dully repetitive and/or irrelevant tasks, you may feel the need to resort to stickers. However, they don't represent a reward: they represent a point on a continuum that runs from apology at one end ("I'm sorry that activity was so mundane") to bribe at the other ("If you complete this mundane activity you'll get a sticker").
b) If the curriculum is relevant and stimulating then pupils will be motivated. This isn't about the National Curriculum, over which we have little control. This is about the curriculum you offer the pupils in your class, the way you frame the National Curriculum in order to make it relevant. If pupils understand how the curriculum is relevant to them or see the subject as stimulating, they'll be engaged (providing the lesson is interesting, purposeful and challenging). You don't need stickers - the curriculum is a reward in itself. If, on the other hand, the pupils are not able to see the relevance of what they're learning about or are unstimulated by the content, you may feel the need to resort to stickers. See above - there's a pattern.
c) If pupils understand their own progress, can see that time in school is impacting on the development of new understanding or skills and have mechanisms to express this (all elements of Assessment for Learning), they will be motivated. Why wouldn't this be so? Picture the disengaged pupils in your class - how many of them can talk positively about the progress that they've made or are making? Progress is its own reward - stickers are redundant. ("You've learnt to do something you couldn't do before. Well done - have a sticker." It makes no sense.) Pupils who are not making progress or who don't perceive or can't articulate progress will become disengaged. You might resort to stickers in the hope....they don't notice they're not learning?
For lots of pupils stickers are part and parcel of how school works. They come to accept the ubiquity of the sticker, and expect achievement or compliance to be rewarded. What makes it worse is that stickers (and other extrinsic rewards) undermine intrinsic motivation.