The core mechanism
When two stocks are not perfectly correlated — that is, they don't move in lockstep — combining them in a portfolio reduces the overall variance of returns relative to holding either alone. The gains of one can partially offset the losses of the other. This is the statistical basis of diversification, first formalised by Harry Markowitz in 1952.
Systematic vs. unsystematic risk
Unsystematic (idiosyncratic) risk is specific to a company or sector — an earnings miss, a regulatory fine, a product recall. Diversification can eliminate this type of risk by spreading across many positions. Systematic (market) risk affects all stocks — a recession, a rate shock, a geopolitical crisis. Diversification cannot eliminate systematic risk.
The free-lunch caveat
Diversification is often called 'the only free lunch in finance' because you can reduce risk without reducing expected return in theory. In practice, adding too many positions increases tracking costs and complexity, and can lead to 'diworsification' — owning so many stocks that gains and losses cancel out regardless of your research.
What Lucex shows
Lucex analyses individual positions — the stock you added to your portfolio — with fundamentals, momentum, and news. It does not assess your portfolio as a whole or make diversification recommendations. Understanding each position in depth is a prerequisite for forming your own view on how they fit together.