Individual supply curves of labour can behave in a variety of ways—over some range they may be positively sloped, over some other ranges they may be negatively sloped or may even be vertical. But a more crucial question is how their sum behaves, i.e., what should be the shape of the market supply curve of any specified type of labour.
Let us first consider the situation in which an industry exclusively uses a specialised type of technically skilled labour. In the short run, as we have just noted, nothing can be said about the individual labour supplies and so nothing can be said about their sum.
It may be positively or negatively sloped or just vertical over a small range in between. But in the long run, we may assume that there is no occupational immobility, and also the young people planning their education and career must surely consider current returns and, what is more important, expected future returns, in various professions.
That is why, in the long run the supply curve of a specialised type of labour would be positively sloped.
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Finally, let us consider the situation in which a particular type of labour is not specialised to any one industry, i.e., more than one industry use a particular type of labour. In this case, the labour supply curve to any one industry must be positively sloped because of the following reasons.
First, to expand employment, any one industry must obtain workers from other industries, thereby increasing the demand price of labour.
Second, the other industries that lose labour would have to reduce their output.
This would increase the commodity prices in these industries and this, in its turn, would exert an upward pressure on the demand price of labour. Thus the industry that attempts to expand employment must face a positively sloped supply curve of labour.