A second income of £1,000 a month from a Stocks and Shares ISA sounds simple enough. Build a large portfolio, reinvest the dividends, and let compounding do the rest.

But turning that idea into reality is far more dependent on how the portfolio is built than many investors realise.

The real driver of second income growth

That £1,000 a month second income from a Stocks and Shares ISA works out at £12,000 a year — implying a portfolio of roughly £300,000 using a 4% income assumption.

On the surface, that makes the goal feel straightforward: build a large enough ISA and the income follows.

But in practice, the path towards that figure can vary dramatically between investors. Small differences in returns and reinvestment can have a surprisingly large impact on how quickly an ISA becomes capable of generating meaningful second income.

The levers behind ISA second income

Rather than viewing second income as a fixed target, it can instead be broken down into three key variables: time, portfolio yield, and ongoing contributions.

Fix any two, and the third becomes the factor that determines how quickly the £1,000-a-month goal is reached.

The table below shows how changing the annual return alone can dramatically shorten the path to a £300,000 portfolio, even with the same £750 monthly contribution.

Annual return	Monthly contribution	Years to target	Portfolio value
4%	£750	21.6	£300,000
6%	£750	18.9	£300,000
8%	£750	16.9	£300,000
10%	£750	15.4	£300,000

This is why stock selection matters. Stronger long-term returns and higher starting yields can materially reduce the time needed for an ISA to begin generating meaningful second income.

Stock picking

For investors trying to accelerate the path towards a meaningful second income, the FTSE 250 contains plenty of businesses still offering sizeable yields.

One that stands out to me is Aberdeen (LSE: ABDN). The asset manager currently offers a dividend yield of around 6.6%, despite the share price climbing roughly 75% over the past year.

I think investors are becoming increasingly optimistic about the company’s prospects. For years, the business struggled with persistent outflows from its key Adviser division. But recent results showed genuine improvement, with net outflows narrowing sharply as the group continues repositioning parts of the business.

What particularly interests me is the longer-term growth opportunity. Demand for self-directed investing and retirement products continues to grow, which plays directly into the company’s interactive investor platform. Customer demand for SIPPs has been especially strong.

At the same time, the group retains deep expertise in Asian and emerging market investing. Those regions lagged badly during the dominance of US mega-cap technology shares, but investor interest is now beginning to broaden into cheaper and less crowded markets.

There are still risks to consider. Asset managers remain heavily exposed to market sentiment, and prolonged weakness in global equities can pressure both flows and earnings. Aberdeen also still needs to prove that recent operational momentum is sustainable.

However, the combination of improving business momentum and a still-generous dividend yield is why I see the shares as one to consider for long-term second income investors.